drm/i915/gt: stop ignoring read with wa_masked_field_set

drm/i915/gt: stop ignoring read with wa_masked_field_set

When using masked registers, there is nothing to clear since a masked
register has the mask in the upper 16b: we can just write to the
location we want and use the mask to control what bits we are writing
to.

However that doesn't mean we don't want to read back the register and
check the value actually matched what we wanted to write, i.e. that
the WA stick. That should be an explicit opt-out for registers that are
either write-only or that are affected by hardware misbehavior.

Moreover both wa_masked_en() and wa_masked_dis() check the WA stick, so
skipping the check just because the field is more than 1 bit is
surprising and error-prone.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20201209045246.2905675-1-lucas.demarchi@intel.com