drm/i915: Rewrite BXT HPD code to conform to pre-existing style

Rewrite the BXT hpd setup to match the way we do it on other platforms:
- Throw out BXT_HOTPLUG_CTL since it's the same as PCH_PORT_HOTPLUG
- Enable the HPD bits in the DE port IER in gen8_de_irq_postinstall()
- Update DE port IMR using bdw_update_port_irq()

Also throw out port D from bxt_port_hotplug_long_detect() since BXT only
goes up to C.

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>