drm/plane-helper: Add drm_plane_helper_check_state()

Add a version of drm_plane_helper_check_update() which takes a plane
state instead of having the caller pass in everything.

And to reduce code duplication, let's reimplement
drm_plane_helper_check_update() in terms of the new function, by
having a tempororary plane state on the stack.

v2: Add a note that the functions modifies the state (Chris)
v3: Fix drm_plane_helper_check_update() y coordinates (Daniel Kurtz)

Cc: Daniel Kurtz <djkurtz@chromium.org>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org> (v2)
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1470642910-14073-1-git-send-email-ville.syrjala@linux.intel.com