drm/i915/gt: Limit VFE threads based on GT

MEDIA_STATE_VFE only accepts the 'maximum number of threads' in the
range [0, n-1] where n is #EU * (#threads/EU) with the number of threads
based on plaform and the number of EU based on the number of slices and
subslices. This is a fixed number per platform/gt, so appropriately
limit the number of threads we spawn to match the device.

v2: Oversaturate the system with tasks to force execution on every HW
thread; if the thread idles it is returned to the pool and may be reused
again before an unused thread.

v3: Fix more state commands, which was causing Baytrail to barf.
v4: STATE_CACHE_INVALIDATE requires a stall on Ivybridge

Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/2024
Fixes: 47f8253d

 ("drm/i915/gen7: Clear all EU/L3 residual contexts")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Prathap Kumar Valsan <prathap.kumar.valsan@intel.com>
Cc: Akeem G Abodunrin <akeem.g.abodunrin@intel.com>
Cc: Jon Bloomfield <jon.bloomfield@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Randy Wright <rwright@hpe.com>
Cc: stable@vger.kernel.org # v5.7+
Reviewed-by: Akeem G Abodunrin <akeem.g.abodunrin@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210111225220.3483-1-chris@chris-wilson.co.uk
(cherry picked from commit eebfb32e

)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>