The video clock is an external resource from the DPSUB point of view,
not a resource internal to the display controller. Move it to the
zynqmp_dpsub structure, to allow accessing it from outside the disp
code.

While at it, rename the fields from pclk and pclk_from_ps to vid_clk and
vid_clk_from_ps, to better reflect their purpose and match the
documentation.
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>