v3dv: don't assume that bound descriptors have been written

v3dv: don't assume that bound descriptors have been written

The pipeline layout lifetime tests in CTS allocate some descriptors
and bind them to the command buffer without actually ever writing
valid resources to them since they never actually execute the command
buffers, so we want to be careful at bind time and not assume the
resources exist.

Fixes crashes in dEQP-VK.api.pipeline.pipeline_layout.lifetime.*

Fixes: 95f881adbd ('v3dv: add support for sampling simple 2D linear textures')
Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25172>