drm/amdkfd: Use kvmalloc instead of kmalloc for VCRAT

Since we're dynamically allocating the CPU VCRAT, use kvmalloc in case
the allocation size is huge.

Signed-off-by: Kent Russell <kent.russell@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>