drm/msm: fix missing NULL check after kcalloc in crashstate_get_bos()

drm/msm: fix missing NULL check after kcalloc in crashstate_get_bos()

The crashstate_get_bos() function allocates memory for `state->bos`
using kcalloc(), but the vmbind path does not check for allocation
failure before dereferencing it in the following drm_gpuvm_for_each_va()
loop. This could lead to a NULL pointer dereference if memory allocation
fails.

Fix this by wrapping the drm_gpuvm_for_each_va() loop with a NULL check
on state->bos, similar to the safety check in the non-vmbind path.

Fixes: af9aa6f316b3d ("drm/msm: Crashdump support for sparse")
Signed-off-by: Huiwen He <hehuiwen@kylinos.cn>
Patchwork: https://patchwork.freedesktop.org/patch/687556/
Signed-off-by: Rob Clark <robin.clark@oss.qualcomm.com>