drm/msm: Add proper checks for GPU LLCC support

Domain attribute setting for LLCC is guarded by !IS_ERR
check which works fine only when CONFIG_QCOM_LLCC=y but
when it is disabled, the LLCC apis return NULL and that
is not handled by IS_ERR check. Due to this, domain attribute
for LLCC will be set even on GPUs which do not support it
and cause issues, so correct this by using IS_ERR_OR_NULL
checks appropriately. Meanwhile also cleanup comment block
and remove unwanted blank line.

Fixes: 00fd44a1 ("drm/msm: Only enable A6xx LLCC code on A6xx")
Fixes: 474dadb8 ("drm/msm/a6xx: Add support for using system cache(LLC)")
Signed-off-by: Sai Prakash Ranjan <saiprakash.ranjan@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>