drm/i915: Rewrite bxt_hpd_handler() to look like everyone else

bxt_hpd_handler() looks different to everyone else for no good reason.
Rewrite it to use the standard variable namees etc.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>