drm/rect: Add DRM_RECT_INIT() macro

Add a helper macro to initialize a rectangle from x, y, width and
height information.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: David Gow <davidgow@google.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: José Expósito <jose.exposito89@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220620160640.3790-2-jose.exposito89@gmail.com