drm/client: Restrict the rotation check to the rotation itself

The drm_client_rotation has a check on the rotation value, but the
reflections are also stored in the same variable, and the check doesn't
take this into account.

Therefore, even though we might have a valid rotation, if we're also using
a reflection parameter, the test will fail for no particular reason.

Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/cf4de0cdef20aac6c654b7b73c2ab3e317c46803.1560783090.git-series.maxime.ripard@bootlin.com