OMAP: DSS2: HDMI: fix hdmi clock name

The HDMI clock (hdmi_clk) is missing in the current OMAP4 HWMOD
database. Fix this in the DSS driver by using the old clock name
(dss_48mhz_clk).

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>