drm/i915: pass intel_crtc as argument for intel_enable_pipe

We want to remove those 3 boolean arguments. This is the first step.
The "pipe" passed as the argument is always intel_crtc->pipe.

Also adjust the function documentation.

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>