drm/i915: Stash hpd status bits under dev_priv

drm/i915: Stash hpd status bits under dev_priv

Instead of constnantly having to figure out which hpd status bit
array to use let's store them under dev_priv.

Should perhaps take this further and stash even more stuff to
make the hpd handling more abstract yet.

v2: Remeber cnp (Imre)
    Add MISSING_CASE() for unknown PCHs (Imre)

Cc: Imre Deak <imre.deak@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200507114808.6150-1-ville.syrjala@linux.intel.com
Reviewed-by: Imre Deak <imre.deak@intel.com>