drm/i915: s/GET_CFG_CR1_REG/DPLL_CFGCR1/ etc.

v2: Use SKL_DPLLx symbolic names instead of raw numbers

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>