amdgpu: increase cpu_map_count storage size

Mesa expects to be able to map the same buffer, without unmapping it.
This leads to problem on long-running program.

On the other hand, libdrm uses cpu_map_count as a refcount and expects
its value to decrease so it can unmap buffers.

The previoulsy proprosed fix (https://patchwork.freedesktop.org/patch/258005/)
stopped increased the counter when it went past INT_MAX.

This commit instead proposes to use a larger type to store cpu_map_count.
The outcome is the same: long running apps will not crash, only the
implementation differs.

Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/1423


Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>