drm/msm/a6xx: Move ioremap out of hw_init path

Move the one-time RPMh setup to a6xx_gmu_init().  To get rid of the hack
for one-time init vs start, add in an extra a6xx_rpmh_stop() at the end
of the init sequence.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Patchwork: https://patchwork.freedesktop.org/patch/527854/
Link: https://lore.kernel.org/r/20230320144356.803762-15-robdclark@gmail.com