drm/fourcc: Clarify the meaning of single-channel "red"

Traditionally, the first channel has been called the "red" channel, but
the fourcc values for single-channel "red" formats can also be used for
other single-channel formats with a direct relationship between channel
value and brightness, like grayscale.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/31d1792f26707a7270347e9916b4103470d2b192.1657294931.git.geert@linux-m68k.org