mesa: Only validate shaders that can exist in the context

mesa: Only validate shaders that can exist in the context

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: Difference at 95.0% confidence 0.495267% +/- 0.202063% (n=40)
64-bit: Difference at 95.0% confidence 3.57576% +/- 0.288175% (n=40)

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