drm/i915/dsi: Configure DSI after enabling DSI pll

We need to enable DSI PLL before configuring the DSI registers.

This has worked before on BYT/CHV, but BXT is more fussy.

Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Tested-by: Mika Kahola <mika.kahola@intel.com> # BXT
Tested-by: Jani Nikula <jani.nikula@intel.com> # BYT
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1454503846-12103-1-git-send-email-ramalingam.c@intel.com