drm/radeon|amdgpu: Remove redundant num_connectors check

The list walk will shortcircuit anyway.

Cc: Alex Deucher <alexdeucher@gmail.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161213230814.19598-4-daniel.vetter@ffwll.ch