drm/i915: Refactor LPT-H VGA dotclock disabling

Extract the LPT-H VGA dotclock disable to a separate function in
anticipation of further use.

While at it move the sb_lock locking inwards when enabling the VGA
dotclock, as it's only needed to protect the sideband accesses.

v2: Keep the PIXCLK_GATE_GATE name for 0 (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/1449260494-14449-1-git-send-email-ville.syrjala@linux.intel.com