gbm_bo_create_with_modifiers doesn't take GBM flags, so our
wlr_gbm_allocator interface doesn't either. We were still internally
using GBM flags in the DRM backend, leading to awkward back-and-forth
conversions.
The only flag passed to drm_plane_init_surface was GBM_BO_USE_LINEAR, so
turn that into a bool to make sure other flags can't be passed in.
Move the "force linear" logic out of init_drm_surface, because the
supplied wlr_drm_format should already contain that information.
93cd3a79b2