drm/i915: add i915_reset_count

reset_counter will be incremented twice per successful
reset. Odd values mean reset is in progress and even values
mean that reset has completed.

Reset status ioctl introduced in following commit
needs to deliver global reset count to userspace so
use reset_counter to derive the actual reset count
for the gpu

Note that reset in progress is enough to increment
the counter.

v2: wedged equals reset in progress (Daniel Vetter)

v3: Fixed stale comments (Damien Lespiau)

Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>