drm/msm/a6xx: add missing MODULE_FIRMWARE()

For platforms that require the "zap shader" to take the GPU out of
secure mode at boot, we also need the zap fw to end up in the initrd.

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