drm/amdgu: fix Unintentional integer overflow for mall size

Potentially overflowing expression mall_size_per_umc * adev->gmc.num_umc with type unsigned int (32 bits, unsigned)
is evaluated using 32-bit arithmetic,and then used in a context that expects an expression of type u64 (64 bits, unsigned).

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