drm/i915/glk: Reuse broxton's cdclk code for GLK

Geminilake has the same register layout, reference clock and programming
sequence as broxton. The difference is that it doesn't support the 1.5
divider and has different ratios, but a lot of code can be shared
between the two platforms.

v2: Rebase (s/broxton/bxt).

v3: Fix vco calculation in glk_de_pll_vco().

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1480667037-11215-7-git-send-email-ander.conselvan.de.oliveira@intel.com