Implement a cm-fx6 specific board_video_skip() to provide the option to not
initialize the display.

The new function does not init display if the environment variable "panel" is
not defined, or if it is set to an unsupported value.

Collateral changes:
- Don't use the global displays array (it's CONFIG_IMX_VIDEO_SKIP specific).
- Don't use detect_hdmi(), since env controlled init makes it unnecessary.

Cc: Stefano Babic <sbabic@denx.de>
Cc: Igor Grinberg <grinberg@compulab.co.il>
Signed-off-by: Nikita Kiryanov <nikita@compulab.co.il>
Signed-off-by: Igor Grinberg <grinberg@compulab.co.il>