fbdev: omapfb: Add 'plane' value check

In the Linux kernel, the following vulnerability has been resolved:

fbdev: omapfb: Add 'plane' value check

Function dispc_ovl_setup is not intended to work with the value OMAP_DSS_WB

of the enum parameter plane.

The value of this parameter is initialized in dss_init_overlays and in the

current state of the code it cannot take this value so it's not a real

problem.

For the purposes of defensive coding it wouldn't be superfluous to check

the parameter value, because some functions down the call stack process

this value correctly and some not.

For example, in dispc_ovl_setup_global_alpha it may lead to buffer

overflow.

Add check for this value.

Found by Linux Verification Center (linuxtesting.org) with SVACE static

analysis tool.