intel: Add support for overriding the PCI ID via an environment variable

For example:

    export INTEL_DEVID_OVERRIDE=0x162

If this variable is set, don't actually submit the batchbuffer to the
GPU, it probably contains commands for the wrong generation of hardware.

v2: Introduce a getter for the overridden devid, and avoid getenv per exec.

Reviewed-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Eric Anholt <eric@anholt.net>