drm/i915/lvds: Remove magic from PLL programming

This looks like a left-over from enabling work. The spec defines
CH7017_LVDS_PLL_FEEDBACK_DEFAULT_RESERVED as reserved set, so follow
this in the programming.

v2:
- Follow the spec to set CH7017_LVDS_PLL_FEEDBACK_DEFAULT_RESERVED.
  (Ville)

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1494408113-379-7-git-send-email-imre.deak@intel.com