drm/i915/debugfs: add crtc i915_pipe debugfs file

The pipe may differ from crtc index if pipes are fused off. For testing
purposes, IGT needs to know the pipe.

There's already a I915_GET_PIPE_FROM_CRTC_ID IOCTL for this. However,
the upcoming Xe driver won't have that IOCTL, and going forward, we'll
want a unified interface for testing i915 and Xe, as they share the
display code. Thus add the debugfs for i915 display.

v2: User letters for pipe names (Ville)

Cc: Bhanuprakash Modem <bhanuprakash.modem@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Tested-by: Bhanuprakash Modem <bhanuprakash.modem@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230320124429.786985-2-jani.nikula@intel.com