drm/i915: Introduce spt_irq_handler()

Starting from SPT the only interrupts living in the south are GMBUS and
HPD. What's worse some of the SPT specific new bits conflict with some
other bits on earlier PCH generations. So better not use the
cpt_irq_handler() for SPT+ anymore.

Also kill the hand rolled port E handling with something more
standardish. This also avoids accidentally confusing port B and port E
long pulses since the bits occupy the same positions, just in different
registers.

Also add a comment noting that the short pulse duration bits are
reserved on LPT+. The 2ms value we program is 0, so no issue wrt. the
MBZ in the spec.

v2: Call intel_hpd_irq_handler() only once (Paulo)

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>