drm/selftest: Refactor drm mode setting selftests

With this patch split the kernel module specific code from actual
selftest code. This is done to allow adding more selftests as separate
file. Also added kernel module exit stub with this patch.

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: alexandru-cosmin.gheorghe@arm.com
Signed-off-by: Deepak Rawat <drawat@vmware.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20181016204609.1555-1-drawat@vmware.com