msm: mdss: Add bounds check in mdss_compat_overlay_ioctl

A bounds check has to be done on ovlist32->num_overlays variable
so as to prevent memory allocation of arbitrary size and avoid
subsequent undetermined behaviour.

Change-Id: Ic70726f1825040e793ef256195d236d77b392791
Signed-off-by: Nirmal Abraham <nabrah@codeaurora.org>
Signed-off-by: Raghavendra Ambadas <rambad@codeaurora.org>
Signed-off-by: Krishna Chaitanya Parimi <cparimi@codeaurora.org>