mesa: Only check for a current vertex shader in core profile

mesa: Only check for a current vertex shader in core profile

This doesn't affect performance, but it feels more correct.

On Bay Trail-D using Fedora 20 compile flags (-m64 -O2 -mtune=generic
for 64-bit and -m32 -march=i686 -mtune=atom for 32-bit), affects
Gl32Batch7:

32-bit: No difference proven at 95.0% confidence (n=120)
64-bit: No difference proven at 95.0% confidence (n=120)

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>