drm/i915: Use DSI Pll1 for enabling MIPI DSI on Port C

DSI Pll1 is used for enabling DSI on Port C.

v2: Addressed review comments of Jani
    - Used & operator instead of == for intel_dsi->ports

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>