backend/drm: take a wlr_drm_format in init_drm_surface
backend/drm: take a wlr_drm_format in init_drm_surface

Instead of taking a format code and wlr_drm_format_set, simplify the
function signature and take a single wlr_drm_format.

SirCmpwn/wlroots,swaywm/wlroots,swaywm/wlroots