drm/format-helper: Rename drm_fb_xrgb8888_to_mono_reversed()

commit db98b3d1e054fbbe61d0be4db1df7bd68d61320f from
https://nv-tegra.nvidia.com/r/3rdparty/canonical/linux-jammy.git jetson_36.3

There is no "reversed" handling in drm_fb_xrgb8888_to_mono_reversed():
the function just converts from color to grayscale, and reduces the
number of grayscale levels from 256 to 2 (i.e. brightness 0-127 is
mapped to 0, 128-255 to 1).  All "reversed" handling is done in the
repaper driver, where this function originated.

Hence make this clear by renaming drm_fb_xrgb8888_to_mono_reversed() to
drm_fb_xrgb8888_to_mono(), and documenting the black/white pixel
mapping.

Fixes: bcf8b616

 ("drm/format-helper: Add drm_fb_xrgb8888_to_mono_reversed()")
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220317081830.1211400-2-geert@linux-m68k.org
(cherry picked from commit 9b13a3fc

)
Change-Id: I0e71ccbfbe31ee996408fb072d73d01bada8ef43
Signed-off-by: Thierry Reding <treding@nvidia.com>
Reviewed-on: https://git-master.nvidia.com/r/c/3rdparty/canonical/linux-jammy/+/2985837
Reviewed-by: svcacv <svcacv@nvidia.com>
Reviewed-by: Jonathan Hunter <jonathanh@nvidia.com>
GVS: Gerrit_Virtual_Submit <buildbot_gerritrpt@nvidia.com>
Signed-off-by: Rajeshkumar Ramasamy <rajeshkumar.ramasamy@windriver.com>