drm/i915/bxt: get DSI pixelclock

BXT's DSI PLL is different from that of VLV. So this patch
adds a new function to get the current DSI pixel clock based
on the PLL divider ratio and lane count.

This function is required for intel_dsi_get_config() function.

v2: Fixed Jani's review comments.

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