Replace supported_formats with is_format_supported interface.

Replace supported_formats with is_format_supported interface.

The old supported_formats interface returned a list of formats
supported by a pipe/winsys implementation. This was reasonable
when gallium had a fixed list of predefined format.
Now things has changed and the definition of PIPE_FORMAT is
more flexible.
The new shiny is_format_supported interface gets PIPE_FORMAT
as an argument and returns a boolean whether this particular
format is supported.