compositor-drm: Simplify drm_sprite_crtc_supported

No need to walk the CRTC list every time looking for CRTC indices, when we
already have the CRTC index stashed away. Taking the plane as an argument
also simplifies things a little for callers, and future-proofs for a
potential future KMS API which passes a list of supported CRTC IDs rather
than a bitmask of supported CRTC indices.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Armin Krezović <krezovic.armin@gmail.com>

Differential Revision: https://phabricator.freedesktop.org/D1407