drm/i915: Rename HSW/BDW PLL bits

Give the PLL control register bits better names on HSW/BDW.

v2: Fix the copy paste fails in SPLL_REF defines (Maarten)

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190610133609.27288-1-ville.syrjala@linux.intel.com
Acked-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com> #irc