drm/i915: Allow the sysadmin to override security mitigations

The clear-residuals mitigation is a relatively heavy hammer and under some
circumstances the user may wish to forgo the context isolation in order
to meet some performance requirement. Introduce a generic module
parameter to allow selectively enabling/disabling different mitigations.

To disable just the clear-residuals mitigation (on Ivybridge, Baytrail,
or Haswell) use the module parameter: i915.mitigations=auto,!residuals

Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/1858


Fixes: 47f8253d ("drm/i915/gen7: Clear all EU/L3 residual contexts")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Jon Bloomfield <jon.bloomfield@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: stable@vger.kernel.org # v5.7
Reviewed-by: Jon Bloomfield <jon.bloomfield@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210111225220.3483-3-chris@chris-wilson.co.uk