drm/i915: Rename 'reg' to 'clk_reg' to unconfuse it from the other 'reg'

On VLV/CHV the rc6 residency calculations read a second register to
determine the actual units used for the residency value. The variable
name 'reg' where that register value is stored shadows the function
argument 'reg'. That can easily leave the reader utterly confused, so
rename the internal variable to 'clk_reg'.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Deepak <S&lt;deepak.s@intel.com>
[danvet: Spellfix in commit message.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>