drm/st7571-i2c: correct pixel data format description

drm/st7571-i2c: correct pixel data format description

The comment describes the pixel data format as stated in
the st7571 datasheet, which is not necessary the same
as for the connected display.

Instead, describe the expected pixel data format which is used for
R1/R2/XRGB8888.

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-v2-1-159f4134098c@gmail.com
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>