OMAP: DSS2: Fix: Return correct lcd clock source for OMAP2/3

dss.lcd_clk_source is set to the default value DSS_CLK_SRC_FCK at dss_init.
For OMAP2 and OMAP3, the dss.lcd_clk_source should always be the same as
dss.dispc_clk_source. The function dss_get_lcd_clk_source() always returns the
default value DSS_CLK_SRC_FCK for OMAP2/3. This leads to wrong clock dumps when
dispc_clk_source is not DSS_CLK_SRC_FCK.

Correct this function to always return dss.dispc_clk_source for OMAP2/3.

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