drm/i915: Fix oops on platforms w/o hpd support

drm/i915: Fix oops on platforms w/o hpd support

We don't have hpd support on i8xx/i915 which means hotplug_funcs==NULL.
Let's not oops when loading the driver on one those machines.

v2: Drop the redundant function pointer check (Jani)

Cc: Dave Airlie <airlied@redhat.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Fixes: 20a7f50d0964 ("drm/i915: constify hotplug function vtable.")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211014090941.12159-5-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>