drm/i915: add media well to VLV force wake routines v2

We could split this out into a separate routine at some point as an
optimization.

v2: use FORCEWAKE_KERNEL (Ville)

Note: Ville mentioned in his review that he declines to be responsible
if this blows up due to the lack of "readback a register != FW_ACK,
but from the same cacheline" magic we have in other forcewake
implementations.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
[danvet: Bikeshed overtly long lines according to checkpatch.pl. Nope,
this time around I didn't screw up printk message since I've left
those alone.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>