drm/client: Change drm_client_panel_rotation name

The drm_client_panel_rotation function has been used so far to set the
default rotation based on the panel orientation.

However, we can have more sources of information to make that decision,
starting with the command line that we will introduce later in this series.

Change the name to remove the panel mention.

Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/8cb0f0d9569d41685bbf30a1538da6578cd2769b.1560783090.git-series.maxime.ripard@bootlin.com