OMAPLFB: Dont foward compositions to DSSCOMP if overlay is Zero

Do not foward compositions to DSSCOMP in case overlay number
passed by HWC is zero. The dsscomp->num_overlay = 0 means
that there is no primary display and no rendering is required.

Change-Id: If7b582efeb55ce84878d0bfbd19eabd2ebd3afaa
Signed-off-by: Tony Lofthouse <a0741364@ti.com>
Signed-off-by: Dandawate Saket <dsaket@ti.com>