fbdev: Make fb_modesetting_disabled() static inline

Make fb_modesetting_disabled() a static-inline function when it is
defined in the header file. Avoid the linker error shown below.

 ld: drivers/video/fbdev/core/fbmon.o: in function `fb_modesetting_disabled':
 fbmon.c:(.text+0x1e4): multiple definition of `fb_modesetting_disabled'; drivers/video/fbdev/core/fbmem.o:fbmem.c:(.text+0x1bac): first defined here

A bug report is at [1].
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Fixes: 0ba2fa8c ("fbdev: Add support for the nomodeset kernel parameter")
Cc: Javier Martinez Canillas <javierm@redhat.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Helge Deller <deller@gmx.de>
Cc: linux-fbdev@vger.kernel.org
Cc: dri-devel@lists.freedesktop.org
Link: https://lore.kernel.org/dri-devel/20221117183214.2473e745@canb.auug.org.au/T/#u # 1
Link: https://patchwork.freedesktop.org/patch/msgid/20221117114729.7570-1-tzimmermann@suse.de

include/linux/fb.h

@@ -807,7 +807,7 @@ extern int fb_find_mode(struct fb_var_screeninfo *var,
 #if defined(CONFIG_VIDEO_NOMODESET)
 bool fb_modesetting_disabled(const char *drvname);
 #else
-bool fb_modesetting_disabled(const char *drvname)
+static inline bool fb_modesetting_disabled(const char *drvname)
 {
 	return false;
 }