drm: omapdrm: Do not use helper unininitialized in omap_fbdev_init()

Clang warns (or errors with CONFIG_WERROR):

  ../drivers/gpu/drm/omapdrm/omap_fbdev.c:235:6: error: variable 'helper' is used uninitialized whenever 'if' condition is true [-Werror,-Wsometimes-uninitialized]
          if (!fbdev)
              ^~~~~~
  ../drivers/gpu/drm/omapdrm/omap_fbdev.c:259:26: note: uninitialized use occurs here
          drm_fb_helper_unprepare(helper);
                                  ^~~~~~
  ../drivers/gpu/drm/omapdrm/omap_fbdev.c:235:2: note: remove the 'if' if its condition is always false
          if (!fbdev)
          ^~~~~~~~~~~
  ../drivers/gpu/drm/omapdrm/omap_fbdev.c:228:30: note: initialize the variable 'helper' to silence this warning
          struct drm_fb_helper *helper;
                                      ^
                                       = NULL
  1 error generated.

Return early, as there is nothing for the function to do if memory
cannot be allocated. There is no point in adding another label to just
emit the warning at the end of the function in this case, as memory
allocation failures are already logged.

Fixes: 3fb1f62f80 ("drm/fb-helper: Remove drm_fb_helper_unprepare() from drm_fb_helper_fini()")
Link: https://github.com/ClangBuiltLinux/linux/issues/1809
Link: https://lore.kernel.org/oe-kbuild-all/202302250058.fYTe9aTP-lkp@intel.com/
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20230224-omapdrm-wsometimes-uninitialized-v1-1-3fec8906ee3a@kernel.org

drivers/gpu/drm/omapdrm/omap_fbdev.c

@@ -233,7 +233,7 @@ void omap_fbdev_init(struct drm_device *dev)
 
 	fbdev = kzalloc(sizeof(*fbdev), GFP_KERNEL);
 	if (!fbdev)
-		goto fail;
+		return;
 
 	INIT_WORK(&fbdev->work, pan_worker);