drm/i915: Disable LPT-H VGA dotclock during crtc disable

Currently we leave the LPT-H VGA dotclock running after turning
the pipe/fdi/port/etc. Properly disable the VGA dotclock as
specified in the modeset sequence.

v2: Fix commit message typo (Paulo)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1449260534-14551-1-git-send-email-ville.syrjala@linux.intel.com