drm/i915/bxt: PORT_PLL_REF_SEL bit should be set for all BXT variations

This patch is to correct one thing in this commit:

commit 25a56705
Author: Dongwon Kim <dongwon.kim@intel.com>
Date:   Wed Mar 16 18:06:13 2016 -0700

    drm/i915/bxt: Reversed polarity of PORT_PLL_REF_SEL bit

This reversed bit polarity is actually common
for all BXT and APL SoCs. Therefore, revision checking
in the original commit should be removed to make
the bit set regardless of revision ID of GFX block.

Signed-off-by: Dongwon Kim <dongwon.kim@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1460673463-14453-1-git-send-email-dongwon.kim@intel.com