drm/i915: Remove nonpriv flags when srm/lrm

On testing the whitelists, using any of the nonpriv
flags when trying to access the register offset will lead
to failure.

Define address mask to get the mmio offset in order
to guard against any current and future flag usage.

v2: apply also on scrub_whitelisted_registers (Lionel)

Cc: Tapani Pälli <tapani.palli@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Signed-off-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20191024110331.8935-1-mika.kuoppala@linux.intel.com