drm/i915: Clarify error message on failed workaround

Let's modify the "workaround lost" error message slightly to make it
more clear what the various numbers represent.  Also, the 'expected'
value needs to be &'d with wa->read so that it doesn't include the mask
bits for masked registers (those bits are write-only in the hardware and
will usually always read out as 0's).

Signed-off-by: Matt Roper <matthew.d.roper@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/20201231191103.854519-1-matthew.d.roper@intel.com