OMAP4: DSSCOMP: Check user passed arg in ioctl

Prevent stack overflow if user passes in a value
that's too large for num_ovl.

Change-Id: I86a232544f8acc6a08eca00dfdb71db41f355341
Signed-off-by: Mike J. Chen <mjchen@google.com>
Signed-off-by: Ruslan Bilovol <ruslan.bilovol@ti.com>