drm/i915: assert_panel_unlocked() in chv_enable_pll()

drm/i915: assert_panel_unlocked() in chv_enable_pll()

Supposedly the power sequencer still locks out the DPLL registers on
CHV, so let's issue a warning if it's still locked when enabling the
DPLL.

Also drop the redundant IS_MOBILE() check for VLV when we check the same
thing.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1458052809-23426-6-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>