drm/tinydrm: Generalize tinydrm_xrgb8888_to_gray8()

drm/tinydrm: Generalize tinydrm_xrgb8888_to_gray8()

This adds parameters for vaddr and clip to tinydrm_xrgb8888_to_gray8() to
make it more generic.

dma_buf_{begin,end}_cpu_access() are moved out to the repaper driver.

Return type is change to void to simplify error handling by callers.

Signed-off-by: David Lechner <david@lechnology.com>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1502127581-10517-2-git-send-email-david@lechnology.com