drm/xe: Remove extra xe_mmio_read32 from xe_mmio_wait32

Commit 5a2be6ba ("drm/xe: Let's return last value read on
xe_mmio_wait32.") mentions that we should return the last value read,
but we never actually return it. This breaks display which depends on
the value being actually returned where needed.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Fixes: 5a2be6ba ("drm/xe: Let's return last value read on xe_mmio_wait32.")
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Closes: #257