i965/skl: Add fast color clear infrastructure

i965/skl: Add fast color clear infrastructure

Patch was originally called:
i965/skl: Enable fast color clears on SKL

Skylake introduces some differences in the way that fast clears are programmed
and in the restrictions for using fast clears. Since some of these are
non-obvious, and fast clears are currently disabled globally, we can enable the
simple stuff here and leave the weirder stuff and separately reviewable work.

Based on a patch originally from Kristian.

Note that within this patch the change in scaling factors could be achieved with
this hunk instead. I've opted to keep things more like how the docs describe it
however.
   --- a/src/mesa/drivers/dri/i965/intel_mipmap_tree.c
   +++ b/src/mesa/drivers/dri/i965/intel_mipmap_tree.c
   @@ -150,9 +150,13 @@ intel_get_non_msrt_mcs_alignment(struct brw_context *brw,
          /* In release builds, fall through */
       case I915_TILING_Y:
          *width_px = 32 / mt->cpp;
   -      *height = 4;
   +      if (brw->gen >= 9)
   +         *height = 2;
   +      else
   +         *height = 4;

v2: Add braces for the multiline (Matt + Chad)
Comment updates (requested by Chad)
Modified commit message
Commit message from Chad explaining the MCS height change (Chad)

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Neil Roberts <neil@linux.intel.com>
Reviewed-by: Chad Versace <chad.versace@intel.com>