defaults/modules_load: add MODULES_DRM

[reagentoo]

[reagentoo]

This PR is done by request https://github.com/gentoo/genkernel/pull/40#issuecomment-1691910552 Current initrd size:

# ls -1sh /boot/ | grep initramfs-6.1.51
 33M initramfs-6.1.51-gentoo-dist.img
 32M initramfs-6.1.51-gentoo-dist.img.old

I'm not sure that we need some additional description in genkernel.conf. But I can add it along with PLYMOUTH option instead of adding MODULES_DRM defaults/modules_load.

[reagentoo]

Full list of available DRM modules for additional approval:

$ find /lib/modules/6.1.51-gentoo-dist/kernel/drivers/gpu/drm -name "*.ko" | xargs basename -a | sort -u | column -c80
amdgpu.ko			gm12u320.ko
analogix-anx78xx.ko		gma500_gfx.ko
analogix_dp.ko			gpu-sched.ko
ast.ko				gud.ko
bochs.ko			hyperv_drm.ko
ch7006.ko			i915.ko
cirrus.ko			ili9163.ko
drm_buddy.ko			ili9486.ko
drm_buddy_test.ko		kvmgt.ko
drm_cmdline_parser_test.ko	mgag200.ko
drm_damage_helper_test.ko	nouveau.ko
drm_display_helper.ko		panel-mipi-dbi.ko
drm_dma_helper.ko		panel-widechips-ws2401.ko
drm_dp_mst_helper_test.ko	qxl.ko
drm_format_helper_test.ko	radeon.ko
drm_format_test.ko		sil164.ko
drm_framebuffer_test.ko		ssd130x-i2c.ko
drm_kms_helper.ko		ssd130x.ko
drm_mipi_dbi.ko			ssd130x-spi.ko
drm_mm_test.ko			ttm.ko
drm_plane_helper_test.ko	udl.ko
drm_rect_test.ko		vgem.ko
drm_shmem_helper.ko		virtio-gpu.ko
drm_ttm_helper.ko		vkms.ko
drm_vram_helper.ko		vmwgfx.ko