staging: sm7xxfb: clean smtcfb_fix's id initialization

Setting up smtcfb_fix's id happens through smtc_alloc_fb_info. It adds
complexity and unnecesary code.

Signed-off-by: Javier M. Mellid <jmunhoz@igalia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>