mesa: Fix bugs in ff fragment shader fog handling

mesa: Fix bugs in ff fragment shader fog handling

This patch fixes two bugs related to fog in the fixed-function
fragment shader generation code.

Fog was only lowered to instructions if MRTs were used.  The fragment
shader assembler always lowers "fog option" code to instructions, and
many drivers (e.g., r300) expect this.

When fog lowering did happen, it was after the instruction count was
checked against implementation limits.  Since fog lowering may add up
to 5 instructions, a program that was below the limits before lowering
may exceed the limits after lowering.

NOTE: This is a candidate for the stable branches.

Reviewed-by: Eric Anholt <eric@anholt.net>
Acked-by: Corbin Simpson <MostAwesomeDude@gmail.com>
Acked-by: Alex Deucher <alexdeucher@gmail.com>