drm: debugfs: Silence warning from cast

Casting a pointer to an integer of a different size generates a warning
from the compiler. First cast the pointer to a pointer-sized type to
keep the compiler happy.

Fixes: 4f66feea ("drm: debugfs: provide infrastructure to dump a DRM GPU VA space")
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/010b58cd-1225-02f7-33f0-c678e5cd349d@arm.com