drm/i915/icl: Gen11 render context size

Gen11 removes the Resource Streamer, which frees up a big chunk of
the context image. BSpec indicates 12544 DWORDs (13 pages), plus
one page for PPHWSP.

Please notice that, when looking at the BSpec context image table,
the right filter has to be applied as some rows are excluded for
specific GENs. Also, some rows apply per-subslice (for the
calculation above, we have supposed I915_MAX_SUBSLICES = 8).

v2: Rebase.
v3: Use the right size as per the BSpec.
v4:
  - Rebased on top of the default context size (Rodrigo)
  - Clarify in the commit message where the subslice calculation
    comes from.
v5: s/12538/12544/ (Daniele)

BSpec: 18907

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Acked-by: Ben Widawsky <benjamin.widawsky@intel.com> (older version)
Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Oscar Mateo <oscar.mateo@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1515711307-28979-2-git-send-email-oscar.mateo@intel.com