drm/i915/display: Rename pipe_timings to transcoder_timings

No functional changes in this patch.

With Bigjoiner, there are 2 pipes driving 2 halfs of 1
transcoder. The transcoder_mode has the full timings, and is used
for configuring the transcoder with the intended mode after
joining the 2 halves.
To clear the confusion, we rename intel_set_pipe_timings to
intel_set_transcoder_timings

v2:
* Split the renaming into separate patch (Ville)

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Reviewed-by: Animesh Manna <animesh.manna@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201008214535.22942-2-manasi.d.navare@intel.com