i965: Silence unused parameter warnings in generated OA code

Reduces my build from 6301 warnings to 2075 warnings by silencing 4226
instances of things like

src/mesa/drivers/dri/i965/i965@sta/brw_oa_hsw.c: In function ‘hsw__render_basic__gpu_core_clocks__read’:
src/mesa/drivers/dri/i965/i965@sta/brw_oa_hsw.c:41:62: warning: unused parameter ‘brw’ [-Wunused-parameter]
 hsw__render_basic__gpu_core_clocks__read(struct brw_context *brw,
                                                              ^~~

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>

src/mesa/drivers/dri/i965/brw_oa.py

@@ -260,7 +260,7 @@ def output_counter_read(set, counter, counter_vars):
 
     c("static " + ret_type)
     read_sym = "{0}__{1}__{2}__read".format(set.get('chipset').lower(), set.get('underscore_name'), counter.get('underscore_name'))
-    c(read_sym + "(struct brw_context *brw,\n")
+    c(read_sym + "(MAYBE_UNUSED struct brw_context *brw,\n")
     c_indent(len(read_sym) + 1)
     c("const struct brw_perf_query_info *query,\n")
     c("uint64_t *accumulator)\n")