drm/msm: Fix error handling crashes seen when VRAM allocation fails

If VRAM allocation fails, the error handling path crashes in
msm_drm_uninit(). The following changes are made to fix this:

msm_gem_shrinker_cleanup() is fixed to unregister the shrinker only
if it was init-ed in the first place.

Before calling kms->funcs->destroy(), we check if kms->funcs is also
non-NULL. This is needed for MDP5, since during msm_drm_int(), priv->kms
becomes non-NULL early, but msm_kms_init() is called on it only later
in mdp5_kms_init().

Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Andy Gross <andy.gross@linaro.org>