drm/i915: implement WaIncreaseDefaultTLBEntries

WaIncreaseDefaultTLBEntries increases the number of TLB
entries available for GPGPU workloads and gives significant
( > 10% ) performance gain for some OCL benchmarks.
Put this in a new function that can be a place for
workarounds that are GT related but not required per ring.
This function is called on driver load and also after a
reset and on resume, so it is safe for workarounds that get
clobbered in these situations. This function currently has
just this one workaround.

v2: This was originally split into 3 patches but following
  review feedback was squashed into 1.
  I have not incorporated some style comments from Chris
  Wilson as I felt that after defining and intialising a
  temporary variable and then adding an additional if block
  to only write the register if the temporary variable had
  been set, this didn't really give a net gain.

v3: Resending in the hope that BAT will run

v4: Change subject line to trigger BAT (please!)

Signed-off-by: Tim Gore <tim.gore@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1454586574-2343-1-git-send-email-tim.gore@intel.com