v3dv/descriptor: return OOM if no descriptor bo when allocating a new descriptor

This could happen if the user created the descriptor pool using a total descriptor count of 0, and then uses it to allocate descriptors using layouts that have descriptors. This is mostly likely an error from the user side, but probably it is better to return OOM instead of crash.

This was found while triaging recent failures with the following tests: dEQP-VK.ycbcr.query.r8g8b8a8_unorm

This mesa patched allowed to get those failures from crash to failure. The final fix was done on the tests themselves (so as mentioned, it was an user error).