drm/i915/icl: Verify engine workarounds in GEN8_L3SQCREG4

Having fixed the incorect MCR programming in an earlier patch, we can now
stop ignoring read back of GEN8_L3SQCREG4 during engine workaround
verification.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190717180624.20354-6-tvrtko.ursulin@linux.intel.com