drm/radeon: fix radeon_atpx_get_client_id()'s return type

drm/radeon: fix radeon_atpx_get_client_id()'s return type

BugLink: http://bugs.launchpad.net/bugs/1782689
The method struct vga_switcheroo_handler::get_client_id() is defined
as returning an 'enum vga_switcheroo_client_id' but the implementation
in this driver, radeon_atpx_get_client_id(), returns an 'int'.

Fix this by returning 'enum vga_switcheroo_client_id' in this driver too.

Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 4a8f264a8a8756bb7d3a478d08e449c67d291ab9)
Signed-off-by: Hui Wang <hui.wang@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>