drm/msm/adreno: Rename gpmufw to powerfw

The power management device on the a5xx cores is known as the
GPMU (Graphics Power Management Unit). On a6xx cores the device
was expanded and renamed as the GMU (Graphics Management Unit).
Rename the 'gpmufw' name struct adreno_info as 'powerfw' to
avoid confusion.

Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>