drm/simpledrm: Enable FB_DAMAGE_CLIPS property

Enable the FB_DAMAGE_CLIPS property to reduce display-update
overhead. Also fixes a warning in the kernel log.

  simple-framebuffer simple-framebuffer.0: [drm] drm_plane_enable_fb_damage_clips() not called

Fix the computation of the blit rectangle. This wasn't an issue so
far, as simpledrm always blitted the full framebuffer. The code now
supports damage clipping and virtual screen sizes.

v3:
	* fix drm_dev_enter() error path (Noralf)
	* remove unnecessary clipping from update function (Noralf)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20211110103702.374-8-tzimmermann@suse.de