drm/msm/a6xx: Remove unneeded GBIF unhalt

Commit e812744c

 ("drm: msm: a6xx: Add support for A618") added a
universal GBIF un-halt into a6xx_start(). This can cause problems for
a630 targets which do not use GBIF and might have access protection
enabled on the region now occupied by the GBIF registers.

But it turns out that we didn't need to unhalt the GBIF in this path
since the stop function already takes care of that after executing a flush
but before turning off the headswitch. We should be confident that the
GBIF is open for business when we restart the hardware.

Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Tested-by: John Stultz <john.stultz@linaro.org>
Reviewed-by: Rob Clark <robdclark@gmail.com>
Fixes: e812744c

 ("drm: msm: a6xx: Add support for A618")
Signed-off-by: Rob Clark <robdclark@chromium.org>