drm/drm_panel: fix export of drm_panel_of_backlight, try #3

drm/drm_panel: fix export of drm_panel_of_backlight, try #3

Making this IS_REACHABLE() was still wrong, as that just determines
whether the lower-level backlight code would be reachable from the panel
driver. However, with CONFIG_DRM=y and CONFIG_BACKLIGHT_CLASS_DEVICE=m,
the drm_panel_of_backlight is left out of drm_panel.o but the condition
tells the driver that it is there, leading to multiple link errors such as

ERROR: "drm_panel_of_backlight" [drivers/gpu/drm/panel/panel-sitronix-st7701.ko] undefined!
ERROR: "drm_panel_of_backlight" [drivers/gpu/drm/panel/panel-sharp-ls043t1le01.ko] undefined!
ERROR: "drm_panel_of_backlight" [drivers/gpu/drm/panel/panel-seiko-43wvf1g.ko] undefined!
ERROR: "drm_panel_of_backlight" [drivers/gpu/drm/panel/panel-ronbo-rb070d30.ko] undefined!
ERROR: "drm_panel_of_backlight" [drivers/gpu/drm/panel/panel-rocktech-jh057n00900.ko] undefined!
ERROR: "drm_panel_of_backlight" [drivers/gpu/drm/panel/panel-panasonic-vvx10f034n00.ko] undefined!
ERROR: "drm_panel_of_backlight" [drivers/gpu/drm/panel/panel-osd-osd101t2587-53ts.ko] undefined!

Change the condition to check for whether the function was actually part
of the drm module. This version of the patch survived a few hundred
randconfig builds, so I have a good feeling this might be the last
one for the export.

Fixes: 4a34a9dcec94 ("drm/drm_panel: Fix EXPORT of drm_panel_of_backlight() one more time")
Fixes: 907aa265fde6 ("drm/drm_panel: fix EXPORT of drm_panel_of_backlight")
Fixes: 152dbdeab1b2 ("drm/panel: add backlight support")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200107203231.920256-1-arnd@arndb.de