Add wrapper for `gbm_device_get_format_modifier_plane_count`

[ids1024]

I'm not sure what to do about the error type here. gbm_dri_get_format_modifier_plane_count does potentially return -1, but doesn't set errno.

Error handling generally looks like it could use some fixes in both gbm.rs and in Mesa...

[yshui]

I found myself needing this as well. What's left to be done here?

[Drakulix]

I'm not sure what to do about the error type here. gbm_dri_get_format_modifier_plane_count does potentially return -1, but doesn't set errno.

Maybe just put this into an Option then and parse every value <= 0 as None? It seems reasonable that some modifiers (e.g. INVALID) don't have a defined plane count.