staging: sm7xxfb: minor maintenance on timing path

This patch keeps smtc_set_timing and sm7xx_set_timing functions closed
to smtcfb_setmode. This change eases reviewing and maintaining this
logic path.

Tested with SM712.

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