st/mesa: Don't record garbage streamout information in the non-SSO case.

In the non-SSO case, where multiple shader stages are linked together,
we were recording garbage pipe_stream_output_info structures for all
but the last enabled geometry-processing stage.

Specifically, we were using the gl_transform_feedback_info from
shader_program->last_vert_prog (the stage whose outputs will be
recorded)...but were pairing it with the output varying mappings
from the current shader stage.  For example, a program with a VS and
GS, the VS's pipe_shader_state would have a pipe_stream_output_info
based on the GS transform feedback info, but the VS output mapping.

This generally worked out okay because only the pipe_stream_output_info
for the last stage really matters - the others can be ignored.  However,
we'd like to avoid confusing the pipe driver.  In particular, my new
driver translates the stream out information to hardware packets at
bind_{vs,tes,gs}_state() time...and was hitting asserts about garbage
varyings that didn't exist.

This patch changes st/mesa to record a blank pipe_stream_output_info
with num_outputs = 0 for all stages prior to last_vert_prog.  The last
one is captured as normal.

(In the fully-SSO case, nothing should change - each program contains
a single shader stage, so last_vert_prog *is* the current shader.)

Tested with llvmpipe (piglit's gpu profile), and freedreno (a3xx,
gpu profile with -t transform.feedback).  Fixes several hundred CTS
tests on my new driver.

Reviewed-by: Timothy Arceri <[email protected]>

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