drm/msm: Fix removal of valid error case when checking speed_bin

Commit 7bf168c8  ("drm/msm: Fix speed-bin support not to
access outside valid memory"), reworked the nvmem reading of
"speed_bin", but in doing so dropped handling of the -ENOENT
case which was previously documented as "fine".

That change resulted in the db845c board display to fail to
start, with the following error:

adreno 5000000.gpu: [drm:a6xx_gpu_init] *ERROR* failed to read speed-bin (-2). Some OPPs may not be supported by hardware

Thus, this patch simply re-adds the ENOENT handling so the lack
of the speed_bin entry isn't fatal for display, and gets things
working on db845c.

Cc: Rob Clark <robdclark@gmail.com>
Cc: Sean Paul <sean@poorly.run>
Cc: Jordan Crouse <jcrouse@codeaurora.org>
Cc: Eric Anholt <eric@anholt.net>
Cc: Douglas Anderson <dianders@chromium.org>
Cc: linux-arm-msm@vger.kernel.org
Cc: freedreno@lists.freedesktop.org
Cc: Bjorn Andersson <bjorn.andersson@linaro.org>
Cc: YongQin Liu <yongqin.liu@linaro.org>
Reported-by: YongQin Liu <yongqin.liu@linaro.org>
Fixes: 7bf168c8  ("drm/msm: Fix speed-bin support not to access outside valid memory")
Signed-off-by: John Stultz <john.stultz@linaro.org>
Reviewed-by: Akhil P Oommen <akhilpo@codeaurora.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Message-Id: <20210330013408.2532048-1-john.stultz@linaro.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>