The DRM device stores a pointer to the fbdev helper. Remove struct
msm_drm_private.fbdev, which contains the same value. No functional
changes.

v2:
	* test for fb_helper->fb in debugfs code
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/530559/
Link: https://lore.kernel.org/r/20230403124538.8497-6-tzimmermann@suse.deSigned-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>