gallium: set PIPE_MAX_SAMPLERS to 18
gallium: set PIPE_MAX_SAMPLERS to 18

For drivers that use higher slots not to crash in tgsi_shader_info.

Reviewed-by: Glenn Kennard <feea5431f8695a3af7470d472cb16ac8e02bab01@gmail.com>
Reviewed-by: Brian Paul <3cb4e1df5ec4da2c7c4af7c52cec8cf340a55a10@vmware.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler