drm/i915: Cleanup SHOTPLUG_CTL status bits definitions

Those status bits don't follow the usual pattern: _MASK (those bits are
write 1 to clear, useful to select the value we want to read) and the
values shifted by the same amount.

Cleaned that that up when poking at the register for testing purposes,
might as well upstream that cleanup.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>