drm/amdgpu: Remove accidentally added small unused hunk.

Commit 8f66090b ("drm/amdgpu: Remove references to struct drm_device.pdev")
accidentally adds an initialization of adev, which isn't used anywhere. This
caused a compiler warning on the unused variable, remove it.

Fixes: 8f66090b ("drm/amdgpu: Remove references to struct drm_device.pdev")
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Christian König <christian.koenig@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>