staging: sm7xxfb: erase hardcode cast between smtcfb_info and fb_info

This patch erases hardcode cast between smtcfb_info and fb_info in order
to get a more robust and less rigid smtcfb_info structure. fb_info
doesn't need to be the first field in smtcfb_info after this patch.

Tested with SM712.

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