drm/st7571-i2c: add support for inverted pixel format

drm/st7571-i2c: add support for inverted pixel format

Depending on which display that is connected to the controller, an
"1" means either a black or a white pixel.

The supported formats (R1/R2/XRGB8888) expects the pixels
to map against (4bit):
    00 => Black
    01 => Dark Gray
    10 => Light Gray
    11 => White

If this is not what the display map against, make it possible to invert
the pixels.

Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Signed-off-by: Marcus Folkesson <marcus.folkesson@gmail.com>
Link: https://lore.kernel.org/r/20250721-st7571-format-v2-4-159f4134098c@gmail.com
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>