drm/client: Add drm_client_framebuffer_flush()

Some drivers need explicit flushing of buffer changes, add a function
that does that.

v2:
- Put all clip rect stuff inside if statement (Sam)

Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200509141619.32970-4-noralf@tronnes.org