OMAP2PLUS: DSS2: Remove hack config "CONFIG_OMAP2_DSS_USE_DSI_PLL"

The clock sources for DISPC_FCLK and LCD1_CLK are now specified in the board file.
There is no need for the hack config "CONFIG_OMAP2_DSS_USE_DSI_PLL" anymore.
Introduce function dpi_use_dsi_pll() which checks for the clock sources to decide
whether DSI PLL is to be used or not.

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