drm/msm: adreno: fix build error without debugfs

The newly added a5xx support fails to build when debugfs is diabled:

drivers/gpu/drm/msm/adreno/a5xx_gpu.c:849:4: error: 'struct msm_gpu_funcs' has no member named 'show'
drivers/gpu/drm/msm/adreno/a5xx_gpu.c:849:11: error: 'a5xx_show' undeclared here (not in a function); did you mean 'a5xx_irq'?

This adds a missing #ifdef.

Fixes: b5f103ab ("drm/msm: gpu: Add A5XX target support")
Cc: stable@vger.kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Rob Clark <robdclark@gmail.com>