drm/format-helper: Streamline blit-helper interface

Move destination-buffer clipping from format-helper blit function into
caller. Rename drm_fb_blit_rect_dstclip() to drm_fb_blit_toio(). Done for
consistency with the rest of the interface. Remove drm_fb_blit_dstclip(),
which isn't required.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20211110103702.374-6-tzimmermann@suse.de