drm/i915: Convert straggling MCHBAR registers

All our registers which are written through the MCHBAR are defined
descriptively as an offset to the MCHBAR. We had 3 outliers here.
Convert these as well so all registers which are offsets are MCHBAR can
be easily identified/found within the code.

With this, convert DCLK to also follow this format.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/i915/intel_pm.c b/drivers/gpu/drm/i915/intel_pm.c
index f313665..2f26d6c 100644
--- a/drivers/gpu/drm/i915/intel_pm.c
+++ b/drivers/gpu/drm/i915/intel_pm.c
@@ -3915,7 +3915,7 @@
 	/* Convert from kHz to MHz */
 	max_ia_freq /= 1000;
 
-	min_ring_freq = I915_READ(MCHBAR_MIRROR_BASE_SNB + DCLK) & 0xf;
+	min_ring_freq = I915_READ(DCLK) & 0xf;
 	/* convert DDR frequency from units of 266.6MHz to bandwidth */
 	min_ring_freq = mult_frac(min_ring_freq, 8, 3);