drm/i915: Clean up the .get_cdclk() assignment if ladder

Let's clean up the mess we have in the if ladder that assigns the
.get_cdclk() hooks. The grouping of the platforms by the function
results in a thing that's not really legible, so let's do it the
other way around and order the if ladder by platform and duplicate
whatever assignments we need.

To further avoid confusion with the function names let's rename
them to just fixed_<freq>_get_cdclk(). The other option would
be to duplicate the functions entirely but it seems quite
pointless to do that since each one just returns a fixed value.

v2: Deal with i945gm_get_cdclk()

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/20170207183226.19537-1-ville.syrjala@linux.intel.com