drm/msm: support firmware-name for zap fw (v2)

Since zap firmware can be device specific, allow for a firmware-name
property in the zap node to specify which firmware to load, similarly to
the scheme used for dsp/wifi/etc.

v2: only need a single error msg when we can't load from firmware-name
    specified path, and fix comment [Bjorn A.]

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