arch/powerpc: Remove <linux/fb.h> from backlight code

Replace <linux/fb.h> with a forward declaration in <asm/backlight.h> to
resolve an unnecessary dependency. Remove pmac_backlight_curve_lookup()
and struct fb_info from source and header files. The function and the
framebuffer struct are unused. No functional changes.

v3:
	* Add Fixes tag (Christophe)
	* fix typos in commit message (Jani)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Fixes: d565dd3b ("[PATCH] powerpc: More via-pmu backlight fixes")
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Michael Ellerman <mpe@ellerman.id.au> # (powerpc)
Link: https://patchwork.freedesktop.org/patch/msgid/20240306122935.10626-4-tzimmermann@suse.de