drm/i915: Enable DSI PLL for both DSI0 and DSI1 in case of dual link

For Dual link MIPI Panels, dsipll clock for both DSI0 and DSI1 needs to be enabled.

v2: Address review comments by Jani
    - Added wait time for PLL to be locked.

v3: separate patch created for cck read for checking PLL to be locked

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