i965: Fall back to normal blorp clear instead of meta clear

i965: Fall back to normal blorp clear instead of meta clear

When intel_miptree_alloc_non_msrt_mcs fails, fall back to normal blorp
color clear instead of falling back to meta.  With this change,
brw_blorp_clear_color can never fail.

v2: Combine two if-statements to remove a level of indentation.
Suggested by Jason.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>