The rotation property should only accept exactly one rotation angle
at once. Let's reject attempts to set none or multiple angles.

Testcase: igt/kms_rotation_crc/bad-rotation
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1474907460-10717-3-git-send-email-ville.syrjala@linux.intel.com