i965: Remove render_target_supported from the vtable.

i965: Remove render_target_supported from the vtable.

brw_render_target_supported() is the only implementation of this
function, so it makes sense to just call it directly.

Rather than adding an #include of brw_wm.h, this patch moves the
prototype to brw_context.h.  Prototypes seem to be in rather arbitrary
places at the moment, and either place seems as good as the other.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Chad Versace <chad.versace@linux.intel.com>