i965: Move brw_context format arrays to intel_screen

i965: Move brw_context format arrays to intel_screen

This allows us to query the driver's supported formats in i965's DRI code,
where often there is available a DRIscreen but no GL context.

To reduce diff noise, this patch does not completely remove
brw_context's format arrays. It just redeclares them as pointers which
point to the arrays in intel_screen.

Specifically, move these two arrays from brw_context to intel_screen:
    mesa_to_isl_render_format[]
    mesa_format_supports_render[]

And add a new array to intel_screen,
    mesa_format_supportex_texture[]
which brw_init_surface_formats() copies to ctx->TextureFormatSupported.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>