drm/i915: Remove BUG_ON call in vlv_enable_pll

Do some further clean up based on the initial review of
drm/i915: Separate cherryview from valleyview.

In this case remove the BUG_ON call in vlv_enable_pll().

v2: Also remove the BUG_ON call in chv_enable_pll(). (Ville)

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Wayne Boyer <wayne.boyer@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1449529362-18193-1-git-send-email-wayne.boyer@intel.com
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>