drm/fb-helper: Allocate shadow buffer of surface height

Allocating a shadow buffer of the height of the buffer object does
not support fbdev overallocation. Use surface height instead.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20211110103702.374-7-tzimmermann@suse.de