drm/i915: Introduce GEN8_DE_PORT_HOTPLUG()

Unify the BDW/BXT hotplug bits. BDW only has port A, but that
matches BXT port A so we can shar the same macro for both.

v2: Remember the gvt

Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201028213323.5423-9-ville.syrjala@linux.intel.com