i965: Remove *_generator::shader field; use prog field instead.

i965: Remove *_generator::shader field; use prog field instead.

The "shader" field in fs_generator, vec4_generator, and gen8_generator
was only used for one purpose; to figure out if we were compiling an
assembly program or a GLSL shader (shader is NULL for assembly
programs).  And it wasn't being used properly: in vec4 shaders we were
always initializing it based on
prog->_LinkedShaders[MESA_SHADER_FRAGMENT], regardless of whether we
were compiling a geometry shader or a vertex shader.

This patch simplifies things by using the "prog" field instead; this
is also NULL for assembly programs.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>