drm/msm: allow zapfw to not be specified in gpulist

For newer devices we want to require the path to come from the
firmware-name property in the zap-shader dt node.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Jordan Crouse <jcrouse@codeaurora.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>