drm/msm/adreno: Fix warn splat for devices without revn

drm/msm/adreno: Fix warn splat for devices without revn

Recently, a WARN_ON() was introduced to ensure that revn is filled before
adreno_is_aXYZ is called. This however doesn't work very well when revn is
0 by design (such as for A635).

Cc: Konrad Dybcio <konrad.dybcio@linaro.org>
Fixes: cc943f43ece7 ("drm/msm/adreno: warn if chip revn is verified before being set")
Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Tested-by: Abhinav Kumar <quic_abhinavk@quicinc.com> # sc7280
Patchwork: https://patchwork.freedesktop.org/patch/545554/