drm/i915: Enable mmio_debug for vlv/chv

With commit 8ac3e1bb ("drm/i915: Add non claimed mmio checking
for vlv/chv") we now have chv/vlv support in place for detecting
unclaimed access. Also the perf hit of extra mmio read
is now only suffered if mmio_debug is set.

This allows us to stuff the macro for unclaimed reg
detection inside a generic gen6 register access, as now all
gens using these macros uses also unclaimed debugs, the one
exception being snb. We gain more clean and generic macros
and only downside is that snb will suffer one branch perf hit
without upside.

Note that the hsw write path debug register check now
happens before fifo check, but this should not make
any real difference.

As vlv/chv use the generic gen6 access macros, the consequence
is that they gain the mmio_debug feature.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1452261080-6979-1-git-send-email-mika.kuoppala@intel.com