drm/msm/a6xx: avoid shadow NULL reference in failure path

If a6xx_hw_init() fails before creating the shadow_bo, the a6xx_pm_suspend
code referencing it will crash. Change the condition to one that avoids
this problem (note: creation of shadow_bo is behind this same condition)

Fixes: e8b0b994

 ("drm/msm/a6xx: Clear shadow on suspend")
Signed-off-by: Jonathan Marek <jonathan@marek.ca>
Reviewed-by: Akhil P Oommen <akhilpo@codeaurora.org>
Link: https://lore.kernel.org/r/20210513171431.18632-6-jonathan@marek.ca
Signed-off-by: Rob Clark <robdclark@chromium.org>