drm/i915: Eliminate HAS_HW_CONTEXTS

HAS_HW_CONTEXTS is misleading condition for GPU reset and CCID,
replace it with Gen specific (to be updated in next patches).

HAS_HW_CONTEXTS in i915_l3_write is bogus because each HAS_L3_DPF
match also has .has_hw_contexts = 1 set.

This leads to us being able to get rid of the property completely.

v2:
- Keep the checks at Gen6 for no functional change (Ville)

Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>