v3dv/cmd_buffer: update shader variants at CmdBindDescriptorSets/CmdBindPipeline

v3dv/cmd_buffer: update shader variants at CmdBindDescriptorSets/CmdBindPipeline

Specially after CmdBindDescriptorSets, it is likely that we would need
a new shader variant, like for example if sampler descriptor sets are
bound.

At that moment a new v3d key is populated, using as base the one used
at pipeline creation, so only cmd_buffer depending values are changed.

Then a new variant is requested. Note that internally it is handled
with a cache, so no new compilation will be done if not needed.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6766>