drm/radeon: change variable name "dev" to "ddev" for consistency

In the probe function of amdgpu, it uses "ddev" as the name of "struct drm_device *",
so I suggest renaming it to be consistent.

Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Tested-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Wu Hoi Pok <wuhoipok@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>