video: tegra: dc: support disp mode override via boot args

Allows overidding the default disp mode without having to recompile
the kernel. Boot args can be specified in the following format:

disp_params=<rgb|hdmi|dsi>:pclk,h_active,v_active,h_ref_to_sync,
v_ref_to_sync,h_sync_width,v_sync_width,h_back_porch,v_back_porch,
h_front_porch,v_front_porch;<rgb|hdmi|dsi>:pclk,h_active,...

Bug 969088

Change-Id: Id0acf608de145493f6749d5b799d4bbb8162ba72
Signed-off-by: Nirav Patel <nipatel@nvidia.com>
Reviewed-on: http://git-master/r/104604
Reviewed-by: Rohan Somvanshi <rsomvanshi@nvidia.com>
Tested-by: Rohan Somvanshi <rsomvanshi@nvidia.com>