drm/amdgpu: integer overflow in amdgpu_mode_dumb_create()

args->size is a u64.  arg->pitch and args->height are u32.  The
multiplication will overflow instead of using the high 32 bits as
intended.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>