OMAP: DSS2: DSI: enable scp clock in dsi_dump_regs()

OMAP4 requires SCP clocks enabled to access DSI PLL registers and DSI COMPLEXIO
registers. Enable scp clock before accessing the registers and disable it before
exiting dsi_dump_regs().

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