OMAP: DSS2: DSI: fix dsi_dump_clocks()

On OMAP4, reading DSI_PLL_CONFIGURATION2 register requires the L3 clock
(CIO_CLK_ICG) to PLL. Currently dsi_dump_clocks() tries to read that
register without enabling the L3 clock, leading to crash if DSI is not
in use.

The status of the bit being read from DSI_PLL_CONFIGURATION2 is
available from dsi_clock_info->use_sys_clk, so we can avoid the whole
problem by just using that.

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