drm/i915: Enable THP on Icelake and beyond

We have a statement from HW designers that the GPU read regression when
using 2M pages was fixed from Icelake onwards, which was also confirmed
by bencharking Eero did last year:

"""
When IOMMU is disabled, enabling THP causes following perf changes on
TGL-H (GT1):

    10-15% SynMark Batch[0-3]
    5-10% MemBW GPU texture, SynMark ShMapVsm
    3-5% SynMark TerrainFly* + Geom* + Fill* + CSCloth + Batch4
    1-3% GpuTest Triangle, SynMark TexMem* + DeferredAA + Batch[5-7]
          + few others
    -7% MemBW GPU blend

In the above 3D benchmark names, * means all the variants of tests with
the same prefix. For example "SynMark TexMem*", means both TexMem128 &
TexMem512 tests in the synthetic (Intel internal) SynMark test suite.

In the (public, but proprietary) GfxBench & GLB(enchmark) test suites,
there are both onscreen and offscreen variants of each test. Unless
explicitly stated otherwise, numbers are for both variants.

All tests are run with FullHD monitor. All tests are fullscreen except
for GLB and GpuTest ones, which are run in 1/2 screen window (GpuTest
triangle is run both in fullscreen and 1/2 screen window).
"""

Since the only regression is MemBW GPU blend, against many more gains,
it sounds it is time to enable THP on Gen11+.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
References: https://gitlab.freedesktop.org/drm/intel/-/issues/430


Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Eero Tamminen <eero.t.tamminen@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220429100414.647857-1-tvrtko.ursulin@linux.intel.com