powerpc: use 'video-mode' environment variable to configure DIU

Use the 'video-mode' environment variable (for Freescale chips that have a
DIU display controller) to designate the full video configuration.  Previously,
the DIU driver used the 'monitor' variable, and it was used only to determine
the output video port.

The old definition of the "monitor" environment variable only determines
which video port to use for output.  This variable was set to a number (0,
1, or sometimes 2) to specify a DVI, LVDS, or Dual-LVDS port.  The
resolution was hard-coded into board-specific code.  The Linux command-line
arguments needed to be hard-coded to the proper video definition string.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Anatolij Gustschin <agust@denx.de>