drm: manager: Fix printk format for size_t

sizeof() returns a size_t which may be different to an unsigned long.
Use the correct format specifier of '%zu' to prevent compiler warnings.

Fixes: e6303f32 ("drm: manager to keep track of GPUs VA mappings")
Reviewed-by: Danilo Krummrich <dakr@redhat.com>
Signed-off-by: Steven Price <steven.price@arm.com>
Signed-off-by: Danilo Krummrich <dakr@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/2bf64010-c40a-8b84-144c-5387412b579e@arm.com