drm/i915: Use enum pipe consistently

Replace all "int pipe"s with "enum pipe pipe"s to make it clear
what we're dealing with.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190821173033.24123-3-ville.syrjala@linux.intel.com


Reviewed-by: Jani Nikula <jani.nikula@intel.com>