drm/rect: Add drm_rect_init()

Add a helper to initialize a rectangle from x/y/w/h information.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190930134214.24702-2-ville.syrjala@linux.intel.com


Reviewed-by: Jani Nikula <jani.nikula@intel.com>