OMAPDSS: DSI: remove option to use pck for DSI PLL clkin

For some OMAP versions the TRM says that the pixel clock from DISPC can
be used as an input clock for DSI PLL, instead of the default, which is
sysclk.  For some OMAP versions the bits affecting this are marked as
reserved.  This feature has never been tested, so it's unknown if the HW
even works, and has never been used.

To clean things up, this patch removes the functionality. This should
not affect any board.

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