drm/i915: Software workaround for getting the HW status of DSI Port C on BYT

Due to hardware limitations on BYT, MIPI Port C DPI Enable bit
does not get set. To check whether DSI Port C was enabled in BIOS,
check the Pipe B enable bit for DSI Port C. In hardware, DSI Port C
is linked with Pipe B.

v2: Addressed review comments of Jani, Nikula
    - Used platform checks for this software workaround for BYT

Signed-off-by: Gaurav K Singh <gaurav.k.singh@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>