drm/gma500: Remove unused helper psb_fbdev_fb_setcolreg()

drm/gma500: Remove unused helper psb_fbdev_fb_setcolreg()

Remove psb_fbdev_fb_setcolreg(), which hasn't been called in almost
a decade.

Gma500 commit 4d8d096e9ae8 ("gma500: introduce the framebuffer support
code") added the helper psb_fbdev_fb_setcolreg() for setting the fbdev
palette via fbdev's fb_setcolreg callback. Later
commit 3da6c2f3b730 ("drm/gma500: use DRM_FB_HELPER_DEFAULT_OPS for
fb_ops") set several default helpers for fbdev emulation, including
fb_setcmap.

The fbdev subsystem always prefers fb_setcmap over fb_setcolreg. [1]
Hence, the gma500 code is no longer in use and gma500 has been using
drm_fb_helper_setcmap() for several years without issues.

Fixes: 3da6c2f3b730 ("drm/gma500: use DRM_FB_HELPER_DEFAULT_OPS for fb_ops")
Cc: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Cc: Stefan Christ <contact@stefanchrist.eu>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Cc: <stable@vger.kernel.org> # v4.10+
Link: https://elixir.bootlin.com/linux/v6.16.9/source/drivers/video/fbdev/core/fbcmap.c#L246
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://lore.kernel.org/r/20250929082338.18845-1-tzimmermann@suse.de