drm/ssd130x: Scale ssd133x per-channel contrast by brightness on init

ssd133x_init() wrote the SSD133X_CONTRAST_A/B/C commands with magic hex
values (0x91/0x50/0x7d).  These are a per-channel white-balance
calibration: the A/B/C channels drive sub-pixels whose OLED materials
differ in luminous efficiency, so the values set the white point at full
brightness.

Extract them into ssd133x_set_contrast(), which scales each channel by a
requested brightness via ssd130x_scale_contrast(), instead of writing the
calibration unconditionally.  This makes the sequence readable, avoids
repetition, and is a prerequisite for wiring up an ssd133x backlight
controller that dims while preserving the white point.

Note this changes the ssd133x power-on brightness.  Previously the init
wrote the calibration unscaled and ignored ssd130x->contrast, so the
panel always booted at full brightness.  It now scales by the shared
default contrast of 127, i.e. half of MAX_CONTRAST (255).  This is
intentional and matches ssd130x, whose contrast register also defaults
to 127 (mid-scale), so all families now power on at ~50% and report
props.brightness = 127 / max_brightness = 255 to userspace.

Assisted-by: Claude:claude-fable-5
Signed-off-by: Amit Barzilai <amit.barzilai22@gmail.com>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patch.msgid.link/20260729053054.29374-2-amit.barzilai22@gmail.com
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>

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