drm/i915/bxt: Remove DSP CLK_GATE programming for BXT

DSP CLK_GATE registers are specific to BYT and CHT.
Avoid programming the same for BXT platform.

v2: Rebased on latest drm nightly branch.

v3: Fixed Jani's review comments

Signed-off-by: Uma Shankar <uma.shankar@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1455796166-13052-1-git-send-email-jani.nikula@intel.com