Better HDMI Flicker-Fix

[sy2002]

Taken from 2ab483fdccdc63ecf9b407cdc3e9ab513dce7cbc:

-- An even better alternative is to cascade two MMCM's: To get from 100 MHz to 31.4496 MHz
-- we need a factor of 0.31449600. Written as a fraction that is 4914/15625 = 2*3^3*7*13 / 5^6.
-- We can factor this fraction into two factors: 2*3*13/5^3 = 0.624 and 3^2*7/5^3 = 0.504.
-- These two factors can we implemented as follows:
-- 0.504 : CLKFBOUT_MULT_F = 47.25, DIVCLK_DIVIDE = 5, CLKOUT0_DIVIDE_F = 18.75
-- 0.624 : CLKFBOUT_MULT_F = 19.50, DIVCLK_DIVIDE = 1, CLKOUT0_DIVIDE_F = 31.25
-- Note: It's necessary that the first MMCM is 0.504, to keep f_VCO of the second MMCM
-- within the range of 600 - 1200 MHz.
-- With the above approach we get the exact clock frequency required, and therefore no
-- longer need any dynamic shifting of phase.

Taken from 6e790078ab01cb5784863ccbc5c4fa1dbddbefd6 which is earlier than the above comment, so the comment above might be the way to go, but I just wanted to ensure that all options are on the table:

-- Note about Flicker-Free: The method used here is to seamlessly (i.e. without glitch)
-- switch automatically between two very close clock speeds. The switching is done based
-- on the feedback from the HDMI ascal'er and is done in mega65.vhd.
--
-- However, there is an alternative method possible, where only a single clock is used and
-- furthermore removes the dependency on the HDMI ascaler. Instead, it uses the "fine
-- phase shift" capability of the MMCM. This makes it possible to dynamically "bend" the
-- clock frequency, but only by a small amount. The calculations are as follows: Starting
-- from the MMCM "i_clk_c64_slow" the actual frequency is 31.44899285 MHz, whereas the
-- desired frequency is 31.449600 MHz.  Since the actual clock is too slow, we need to
-- "insert" extra clock cycles. We do this by occasionally shortening a clock cycle by a
-- small amount.  The number of clock cycles before we "insert" a complete extra clock
-- cycle is: 31.449600 / (31.449600 - 31.44899285) = 51799.  Given the configuration
-- values of the MMCM there is a total of 56*21.375 = 1197 units of "fine phase shift" in
-- each clock cycle.  Therefore, we need to remove one unit of phase shift every
-- 51799/1197 = 43.27 clock cycles.