drm/format-helper: Add drm_fb_xrgb8888_to_mono_reversed()

Add support to convert from XR24 to reversed monochrome for drivers that
control monochromatic display panels, that only have 1 bit per pixel.

The function does a line-by-line conversion doing an intermediate step
first from XR24 to 8-bit grayscale and then to reversed monochrome.

The drm_fb_gray8_to_mono_reversed_line() helper was based on code from
drivers/gpu/drm/tiny/repaper.c driver.

Signed-off-by: Javier Martinez Canillas <[email protected]>
Reviewed-by: Thomas Zimmermann <[email protected]>
Reviewed-by: Andy Shevchenko <[email protected]>
Reviewed-by: Maxime Ripard <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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