drm/radeon: remove unneeded header include path

A header include path without $(srctree)/ is suspicious because it does
not work with O= builds.

You can build drivers/gpu/drm/radeon/ without this include path.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>