drm/i915: Introduce i915_has_asle()

drm/i915: Introduce i915_has_asle()

We want to allow the desktop PNV to not have .is_mobile set. To
that end let's add a small helper to determine if the platform
has the ASLE interrupt (or equivalent). Supposdely both PNV
variants have it.

Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190318165633.28924-4-ville.syrjala@linux.intel.com
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>