drm/i915: fix PLL assertions for DSI PLL

For DSI, we need to be asserting DSI PLL, not DPLL.

This is a somewhat stopgap implementation. It's slightly ugly to have to
pass the dsi parameter to intel_enable_pipe().

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>