drm/i915: Remove per-platform IIR HPD masking

We no longer unmask all HPD irqs, so we can drop the ugly per-platform
HPD IIR masking. IMR will prevent unsupported bits from appearing in
IIR.

v2: Deal with DG1
    Include "HOTPLUG" in the mask names (Lucas)
v3: Fix typos in subject

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/20201028213323.5423-17-ville.syrjala@linux.intel.com