gbm: Add a gbm_device_get_format_modifier_plane_count function

gbm: Add a gbm_device_get_format_modifier_plane_count function

This allows the user to query the number of planes required by a given
format+modifier combination without having to create a bo or surface.

Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>