drm/selftest: Refactor test-drm_plane_helper

The idea is to split test implementations in different compilation
units, but have one single place where we define the list of tests,
in this case(drm_modeset_selftests.h).

Signed-off-by: Alexandru Gheorghe <alexandru-cosmin.gheorghe@arm.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20181019105752.17741-9-alexandru-cosmin.gheorghe@arm.com