drm/drm_panel: fix EXPORT of drm_panel_of_backlight

Fix link failure for module builds of panels.
The conditional compilation around drm_panel_of_backlight()
was wrong for a module build.
Fix it using IS_ENABLED().

Signed-off-by: Sam Ravnborg <[email protected]>
Reviewed-by: Linus Walleij <[email protected]>
Fixes: 152dbdeab1b2 ("drm/panel: add backlight support")
Cc: Sam Ravnborg <[email protected]>
Cc: Laurent Pinchart <[email protected]>
Cc: Thierry Reding <[email protected]>
Cc: Maarten Lankhorst <[email protected]>
Cc: Maxime Ripard <[email protected]>
Cc: Sean Paul <[email protected]>
Cc: David Airlie <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: Maxime Ripard <[email protected]>
Cc: [email protected]
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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