i965/gen6: Fix crash with VS+TF after rendering with GS

i965/gen6: Fix crash with VS+TF after rendering with GS

Rendering with a GS and then using transform feedback with a program that does
not have a GS can crash in gen6. The reason for this is that
brw_begin_transform_feedback checks brw->geometry_program to decide if there
is a GS program, but this is not correct: brw->geometry_program is updated when
issuing drawing commands, so after rendering with a GS it will be non-NULL
until we draw again with a program that does not have a GS. If the next
program uses TF, we will call glBegintransformFeedback before issuing
the drawing command and hence brw->geometry_program will be non-NULL if
the previous rendering used a GS. The right thing to do here is to check
ctx->_Shader->CurrentProgram[MESA_SHADER_GEOMETRY] instead. This is what the
gen7 code path does too.

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=87694
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>