regulator: rpi-panel: Remove get_brightness hook

commit 1d746d44

 upstream.

The driver was implementing a get_brightness function that
tried to read back the PWM setting of the display to report
as the current brightness.
The controller on the display does not support that, therefore
we end up reporting a brightness of 0, and that confuses
systemd's backlight service.

Remove the hook so that the framework returns the current
brightness automatically.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Signed-off-by: Detlev Casanova <detlev.casanova@collabora.com>
Link: https://lore.kernel.org/r/20220124220129.158891-8-detlev.casanova@collabora.com


Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Rahul T R <r-ravikumar@ti.com>