drm/msm: Upgrade gxpd checks to IS_ERR_OR_NULL

dev_pm_domain_attach_by_name() can return NULL, so we should check for
that case when we're about to dereference gxpd.

Fixes: 9325d426

 ("drm/msm/gpu: Attach to the GPU GX power domain")
Cc: Jordan Crouse <jcrouse@codeaurora.org>
Cc: Rob Clark <robdclark@chromium.org>
Reviewed-by: Jordan Crouse <jcrouse@codeauorora.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190515170104.155525-1-sean@poorly.run