drm: of: Export and rename drm_crtc_port_mask()

Function is useful when drm_of_find_possible_crtcs() can't be used and
custom parsing is needed. This can happen for example when there is a
node with multiple muxes between crtc and encoder.

Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
[maxime: change the function to have a consistent prefix]
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180625120304.7543-22-jernej.skrabec@siol.net