i965: Expose logic telling if non-msrt mcs is supported

i965: Expose logic telling if non-msrt mcs is supported

Alos use the opportunity to mark inputs constant. (Context has to be
given as read-write to intel_miptree_supports_non_msrt_fast_clear()
to support debug output).

Signed-off-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
Reviewed-by: Ben Widawsky <benjamin.widawsky@intel.com>