drm/i915: s/get_display_clock_speed/get_cdclk/

Rename the .get_display_clock_speed() hook to .get_cdclk().
.get_cdclk() is more specific (which clock) and it's much
shorter.

v2: Deal with IS_GEN9_BC()
v3: Deal with i945gm_get_display_clock_speed()

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ander Conselvan de Oliveira <conselvan2@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170207183146.19420-1-ville.syrjala@linux.intel.com