drm/msm: fix building without debugfs

The adreno driver stopped building when CONFIG_DEBUGFS is disabled:

drivers/gpu/drm/msm/adreno/adreno_device.c: In function 'adreno_load_gpu':
drivers/gpu/drm/msm/adreno/adreno_device.c:153:16: error: 'const struct msm_gpu_funcs' has no member named 'debugfs_init'
  if (gpu->funcs->debugfs_init) {
                ^~
drivers/gpu/drm/msm/adreno/adreno_device.c:154:13: error: 'const struct msm_gpu_funcs' has no member named 'debugfs_init'
   gpu->funcs->debugfs_init(gpu, dev->primary);
             ^~

This adds an #ifdef around the code that references the hidden
pointer.

Fixes: 331dc0bc ("drm/msm: add a5xx specific debugfs")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Rob Clark <robdclark@gmail.com>