drm/i915: abstract away platform specific parts from hpd handling

Split intel_hpd_irq_handler into platforms specific and platform
agnostic parts. The platform specific parts decode the registers into
information about which hpd pins triggered, and if they were long
pulses. The platform agnostic parts do further processing, such as
interrupt storm mitigation and scheduling bottom halves.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>