OMAP: DSS2: Clean up stallmode and io pad mode selection

Split the function dispc_set_parallel_interface_mode() into 2 separate
functions called dispc_mgr_set_io_pad_mode() and dispc_mgr_enable_stallmode().
The current function tries to set 2 different modes(io pad mode and stall mode)
based on a parameter omap_parallel_interface_mode which loosely corresponds to
the panel interface type.

This isn't correct because a) these 2 modes are independent to some extent,
b) we are currently configuring gpout0/gpout1 for DSI panels which is
unnecessary, c) a DSI Video mode panel won't get configured correctly.

Splitting the functions allows the interface driver to set these modes
independently and hence allow more flexibility.

Signed-off-by: Archit Taneja <archit@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>