drm/panel: st7701: Fix AVCL calculation

drm/panel: st7701: Fix AVCL calculation

The AVCL register, according to the datasheet, comes in increments
of -0.2v between -4.4v (represented by 0x0) to -5.0v (represented
by 0x3). The current calculation is done by adding the defined
AVCL value in mV to -4400 and then dividing by 200 to get the register
value. Unfortunately if I subtract -4400 from -4400 I get -8800, which
divided by 200 gives me -44. If I instead subtract -4400 from -4400
I get 0, which divided by 200 gives me 0. Based on the datasheet this
is the expected register value.

Fixes: 83b7a8e7e88e ("drm/panel/panel-sitronix-st7701: Parametrize voltage and timing")
Signed-off-by: Chris Morgan <macromorgan@hotmail.com>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://lore.kernel.org/r/20231208154847.130615-2-macroalpha82@gmail.com
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20231208154847.130615-2-macroalpha82@gmail.com