drm/i915/bxt: DSI disable and post-disable

This patch contains changes to support DSI disble sequence in BXT.
The changes are:
1. BXT specific changes in clear_device_ready function.
2. BXT specific changes in DSI disable and post-disable functions.
3. Add a new function to reset BXT Dphy clock and dividers
   (bxt_dsi_reset_clocks).
4. Moved some part of the vlv clock reset code, in a new function
   (vlv_dsi_reset_clocks) maintaining the exact same sequence.
5. Wrapper function to call corresponding reset clock function.

v2: Fixed Jani's review comments.

v3: Removed the GET_DSI_PORT_CTRL Macro for consistency with earlier
    implementations as per Jani's suggestion.

Signed-off-by: Uma Shankar <uma.shankar@intel.com>
Signed-off-by: Shashank Sharma <shashank.sharma@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>