drm/i915: fix build issue when using clang

drm/i915 adds some extra cflags, namely -Wall, which causes instances of
-Wformat-security to appear when building with clang, even though this
warning is turned off kernel-wide in the main Makefile:

drivers/gpu/drm/i915/gt/intel_gt.c:983:2: error: format string is not a string literal (potentially insecure) [-Werror,-Wformat-security]
        GEM_TRACE("ERROR\n");
        ^~~~~~~~~~~~~~~~~~~~
./drivers/gpu/drm/i915/i915_gem.h:76:24: note: expanded from macro 'GEM_TRACE'
 #define GEM_TRACE(...) trace_printk(__VA_ARGS__)
                       ^~~~~~~~~~~~~~~~~~~~~~~~~
./include/linux/kernel.h:369:3: note: expanded from macro 'trace_printk'
                do_trace_printk(fmt, ##__VA_ARGS__);    \
                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
./include/linux/kernel.h:383:30: note: expanded from macro 'do_trace_printk'
                __trace_bprintk(_THIS_IP_, trace_printk_fmt, ##args);   \
                                          ^~~~~~~~~~~~~~~~
drivers/gpu/drm/i915/gt/intel_gt.c:983:2: note: treat the string as an argument to avoid this

This does not happen with GCC because it does not enable
-Wformat-security with -Wall. Disable -Wformat-security within the i915
Makefile so that these warnings do not show up with clang.
Signed-off-by: Tong Zhang <ztong0001@gmail.com>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220214195821.29809-1-ztong0001@gmail.com

drivers/gpu/drm/i915/Makefile

@@ -13,6 +13,7 @@
 # will most likely get a sudden build breakage... Hopefully we will fix
 # new warnings before CI updates!
 subdir-ccflags-y := -Wall -Wextra
+subdir-ccflags-y += -Wno-format-security
 subdir-ccflags-y += -Wno-unused-parameter
 subdir-ccflags-y += -Wno-type-limits
 subdir-ccflags-y += -Wno-missing-field-initializers