radv: enable FMASK for color attachments only

radv: enable FMASK for color attachments only

The reason behind this is that FMASK requires CMASK and also that
FMASK for non color attachments looks unnecessary. It's currently
much easier to add this simple check because the driver tries to
always enable DCC first and if we enable FMASK only if CMASK, we
might loose some FMASK compressions.

This helps fixing some new robustness2 tests which fails because
only FMASK is enabled.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4783>