drm/i915/gt: Set BLIT_CCTL reg to un-cached

Blitter commands which do not have MOCS fields rely on
cacheability of BlitterCacheControlRegister which was mapped
to index 0 by default.Once we changed the MOCS value of
index 0 to L3 WB, tests like gem_linear_blits started failing
due to a change in cacheability from UC to WB.

Program and place the BlitterCacheControlRegister in
build_aux_regs().

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Ayaz A Siddiqui <ayaz.siddiqui@intel.com>
Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210903092153.535736-4-ayaz.siddiqui@intel.com