drm/msm: don't crash if no msm.vram param

If VRAM carveout is used, due to no IOMMU, we should have a default
value for msm.vram so that we don't simply crash.

Reported-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>