drm/i915: Replace bxt_verify_cdclk_state() with a more generic cdclk check

Rather than having a BXT specific function to make sure the DE PLL is
enabled after disabling DC6, let's just make sure the current cdclk
is the same as what we last programmed.

Having another check in bxt_display_core_init() almost immediately after
the cdclk init seems redundant, so let's just kill that one.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1463172100-24715-21-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Imre Deak <imre.deak@intel.com>