OMAP: DSS2: add dss_get_hdmi_venc_clk_source()

Add dss_get_hdmi_venc_clk_source(), which can be used to get the value
programmed with dss_select_hdmi_venc_clk_source(). This can be used to
find out if the digit output is going to VENC or HDMI.

For OMAP2/3 dss_get_hdmi_venc_clk_source() always returns
DSS_VENC_TV_CLK.

Cc: Mythri P K <mythripk@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>