drm/msm/a6xx: Rename a6xx_gmu_probe to a6xx_gmu_init

This rename makes it more clear that everything initialized in the _init
function must be cleaned up in a6xx_gmu_remove. This will hopefully
dissuade people from using device managed resources (for reasons laid
out in the previous patch).

Changes in v2:
- None

Cc: Jordan Crouse <jcrouse@codeaurora.org>
Reviewed-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190523171653.138678-6-sean@poorly.run