drm/format-helper: Add drm_fb_xrgb8888_to_gray8_line()

Pull the per-line conversion logic into a separate helper function.

This will allow to do line-by-line conversion in other helpers that
convert to a gray8 format.

Suggested-by: Thomas Zimmermann <[email protected]>
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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