drm/i915: Add NEEDS_FORCEWAKE() checks for vlv/chv

drm/i915: Add NEEDS_FORCEWAKE() checks for vlv/chv

Include an early NEEDS_FORCEWAKE() check for vlv and chv.
Hopefully that will avoid doing so many range checks in for many
register accesses (at least for all display registers).

Note that vlv already had the check in the write path since it shares
the gen6+ code for that.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1445517300-28173-6-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>