gallium: remove PIPE_MAX_VERTEX/GEOMETRY_SAMPLERS #define

gallium: remove PIPE_MAX_VERTEX/GEOMETRY_SAMPLERS #define

PIPE_MAX_SAMPLERS, PIPE_MAX_VERTEX_SAMPLERS and PIPE_MAX_GEOMETRY_SAMPLERS
were all defined to the same value (16).

In various places we're creating arrays such as
sampler_views[PIPE_SHADER_TYPES][PIPE_MAX_SAMPLERS] so we were assuming
the same number of max samplers for all shader stages anyway.

Of course, drivers are still free to advertise different numbers of max
samplers for different shaders.