GCC diagnostic pragma method throws below warnings in some of the versions

drivers/gpu/drm/msm/adreno/a6xx_gpu_state.c:16:9: warning: unknown
option after '#pragma GCC diagnostic' kind [-Wpragmas]
  #pragma GCC diagnostic ignored "-Wunused-const-variable"
          ^
In file included from drivers/gpu/drm/msm/adreno/a6xx_gpu_state.c:18:0:
drivers/gpu/drm/msm/adreno/adreno_gen7_0_0_snapshot.h:924:19: warning:
'gen7_0_0_external_core_regs' defined but not used [-Wunused-variable]
  static const u32 *gen7_0_0_external_core_regs[] = {
                    ^
In file included from drivers/gpu/drm/msm/adreno/a6xx_gpu_state.c:19:0:
drivers/gpu/drm/msm/adreno/adreno_gen7_2_0_snapshot.h:748:19: warning:
'gen7_2_0_external_core_regs' defined but not used [-Wunused-variable]
  static const u32 *gen7_2_0_external_core_regs[] = {
                    ^
In file included from drivers/gpu/drm/msm/adreno/a6xx_gpu_state.c:20:0:
drivers/gpu/drm/msm/adreno/adreno_gen7_9_0_snapshot.h:1188:43: warning:
'gen7_9_0_sptp_clusters' defined but not used [-Wunused-variable]
  static struct gen7_sptp_cluster_registers gen7_9_0_sptp_clusters[] = {
                                            ^
drivers/gpu/drm/msm/adreno/adreno_gen7_9_0_snapshot.h:1438:19: warning:
'gen7_9_0_external_core_regs' defined but not used [-Wunused-variable]
  static const u32 *gen7_9_0_external_core_regs[] = {

Remove GCC version dependency by using __unused__ for the unused gen7_* includes.

Changes in v2:
	- Fix the warnings in the commit text
	- Use __attribute((__unused__)) instead of local assignment

changes in v3:
	- drop the Link from the auto add

changes in v4:
	- replace __attribute((__unused__)) with __always_unused

Fixes: 64d62556 ("drm/msm: More fully implement devcoredump for a7xx")
Suggested-by: Rob Clark <robdclark@chromium.org>
Signed-off-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/597265/
[Add gen7_9_0_cx_debugbus_blocks as well]
Signed-off-by: Rob Clark <robdclark@chromium.org>