drm: Add macro to export functions only when CONFIG_DRM_DEBUG_SELFTEST is enabled

If we want to be able to write drmselftests for non-static core
functions that are not intended to be used by drivers we need this
functions to be exported.

This adds a macro that is tied of CONFIG_DRM_DEBUG_SELFTEST, and uses
that to export drm_internal_framebuffer_create, in order for
subsequent patches to be able to test it.

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/20181101151051.1509-7-alexandru-cosmin.gheorghe@arm.com