drm: add drm_fb_xrgb8888_to_rgb565_dstclip()

It is a drm_fb_xrgb8888_to_rgb565() variant which checks the clip
rectangle for the destination too.

Common code between drm_fb_xrgb8888_to_rgb565() and
drm_fb_xrgb8888_to_rgb565_dstclip() was factored out into the
drm_fb_xrgb8888_to_rgb565_lines() helper function.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20190405095219.9231-4-kraxel@redhat.com