Remove clock constraints related to passive matrix displays.

There is a constraint (pcd_min should be 3) for passive matrix displays. Remove
this constraint in clock divider calculations as we won't support passive
matrix displays any more.

This cleans up the functions which calculate the clock dividers with DSI's PLL
or DSS_FCLK as the clock source.

Signed-off-by: Archit Taneja <archit@ti.com>