drm/i915: fix assert_cursor on BDW

We need to read the correct register, not a register that doesn't exist
and will trigger "Unclaimed register" messages when we touch it.

Also rearrange the checks in an attempt to prevent this error from
happening again.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
[Jani: dropped an extra empty line introduced.]
Signed-off-by: Jani Nikula <jani.nikula@intel.com>