st/mesa: support GL_ARB_get_program_binary even disk cache is disabled
Current implementation of GL_ARB_get_program_binary is depended on disk cache support of specific gallium driver.
GL_ARB_get_program_binary is not supported if get_disk_shader_cache is not implemented in the specific gallium driver. GL_ARB_get_program_binary is either not supported if ENABLE_SHADER_CACHE is disabled.
This patch will break the relationship of the two things, then we can support GL_ARB_get_program_binary even get_disk_shader_cache is NULL or ENABLED_Shader_Cache is disabled.