drm/i915: fix header test with GCOV

$(CC) with $(CFLAGS_GCOV) assumes the output filename with .gcno suffix
appended is writable. This is not the case when the output filename is
/dev/null:

  HDRTEST drivers/gpu/drm/i915/display/intel_frontbuffer.h
/dev/null:1:0: error: cannot open /dev/null.gcno
  HDRTEST drivers/gpu/drm/i915/display/intel_ddi.h
/dev/null:1:0: error: cannot open /dev/null.gcno
make[5]: *** [../drivers/gpu/drm/i915/Makefile:307:
drivers/gpu/drm/i915/display/intel_ddi.hdrtest] Error 1
make[5]: *** Waiting for unfinished jobs....
make[5]: *** [../drivers/gpu/drm/i915/Makefile:307:
drivers/gpu/drm/i915/display/intel_frontbuffer.hdrtest] Error 1

Filter out $(CFLAGS_GVOC) from the header test $(c_flags) as they don't
make sense here anyway.

References: http://lore.kernel.org/r/d8112767-4089-4c58-d7d3-2ce03139858a@infradead.org
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Fixes: c6d4a099

 ("drm/i915: reimplement header test feature")
Cc: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200221105414.14358-1-jani.nikula@intel.com
(cherry picked from commit 408c1b32

)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>