OMAPDSS: HDMI: Discard phy_tx_enabled member

It is simpler to read the current status from a register as compared
to maintaining a state variable to hold the information.

Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>