drm/selftests: Add tests for drm_internal_framebuffer_create

Add tests that verify that drm_internal_framebuffer_create creates
buffers correctly by creating a dummy drm_device with a mock function
for the fb_create callback.

To decide if a buffer has been created or not it just checks if
fb_create callback has been called for the particular drm_mode_fb_cmd2
that's being tested.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Alexandru Gheorghe <alexandru-cosmin.gheorghe@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181101151051.1509-8-alexandru-cosmin.gheorghe@arm.com