sunxi: video: select dw-hdmi in Kconfig, not Makefile

Currently sunxi Makefile manually specifies full path to dw-hdmi common
code. However, that is not needed because it can be selected in Kconfig
instead.

Select proper symbol in Kconfig and drop path from Makefile.

Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>