OMAPDSS: DISPC: Improvements to DIGIT sync signal selection

DSS code wrongly assumes that VENC is always available as source for the external
sync signal for the display controller DIGIT channel. One cannot blindly write/read
the value of DSS_CONTROL[15] as in certain processors (e.g., OMAP5) this operation
may not be valid. If the the sync source is not read correctly, the callers of
dss_get_hdmi_venc_clk_source might make wrong assumptions about, for instance,
video timings.

Logic is added to correctly get the sync signal based on the available displays
in the DIGIT channel. The source is set only if both VENC and HDMI are supported.

Signed-off-by: Ricardo Neri <ricardo.neri@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>