drm/atomic: Reject attempts to use multiple rotation angles at once

drm/atomic: Reject attempts to use multiple rotation angles at once

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 <[email protected]>
Reviewed-by: Joonas Lahtinen <[email protected]>
Reviewed-by: Chris Wilson <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]