gallium: Introduce separate vertex texture/sampler state.

gallium: Introduce separate vertex texture/sampler state.

Previously, gallium shared sampler and texture state between vertex
and fragment shader stages. This change generalises this concept by
providing separate entrypoints for vertex and fragment sampler state
setting.

A new capability bit is added to query the driver for the number
of samplers that can be utilised by a vertex and fragment shader
at the same time.