drm/i915/dp: Extend BDW DP audio workaround to GEN9 platforms

According to BSpec, cdclk for BDW has to be not less than 432 MHz with DP
audio enabled, port width x4, and link rate HBR2 (5.4 GHz). With cdclk less
than 432 MHz, enabling audio leads to pipe FIFO underruns and displays
cycling on/off.

Let's apply this work around to GEN9 platforms too, as it fixes the same
issue.

v2: Move drm_device to drm_i915_private conversion

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=97907
Cc: stable@vger.kernel.org
Cc: Libin Yang <libin.yang@linux.intel.com>
Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1478117601-19122-1-git-send-email-dhinakaran.pandiyan@intel.com
(cherry picked from commit 9c754024

)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>