OMAP: DSS2: DPI: remove unneeded SYSCK enable/disable

DSI PLL requires sys_clk to function, and DPI enables sys_clk when it
wants to use DSI PLL. However, DSI PLL code already handles enabling
sys_clk, so DPI's sys_clk code is extra.

Remove the unneeded sys_clk handling from dpi.c.

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