drm/i915: remove "dsi" argument form intel_enable_pipe

Now that we pass struct intel_crtc as an argument, we can check for
DSI inside the function, removing one more of those confusing boolean
arguments.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>