uvesafb: provide option to specify timeout for task completion

We try to make this change a generic extension, but it is
actually for a corner case. When a VM (qemu) gets a very limited
cpu bandwidth from host, which could be under a heavy load, the
existing 5000 ms timeout could occur and trigger error messages
in the task function's callers.

This change adds a new timeout parameter so that we can tweak
the value as a workaround or for troubleshooting purposes. In
the infinite wait case, A warning message is printed at 5000ms
interval.

In real world, the current 5 sec is generous enough for a video
request in my opinion, so this change could not be very useful.

Upstream Status: Inappropriate

Signed-off-by: Jianxun Zhang <jianxun.zhang@linux.intel.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>