OMAPDSS: n8x0 panel: remove use of platform_enable/disable

The n8x0 panel driver now manages the gpios required to configure the panel.
This was previously done in panel_n8x0_data's platform_enable/disable callbacks
defined in board files using this panel.

All the board files using this panel now pass the gpio information as platform
data via the panel_n8x0_data struct, which is needed by the panel driver to
configure the gpios connected to the panel. Hence, the uses of
platform_enable/disable ops can be safely removed now.

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