drm/amdgpu: Fix integer overflow in amdgpu_cs_pass1

The type of size is unsigned int, if size is 0x40000000, there will
be an integer overflow, size will be zero after size *= sizeof(uint32_t),
will cause uninitialized memory to be referenced later.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: hackyzh002 <hackyzh002@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>