i965: Disable shader cache with INTEL_DEBUG=shader_time

Shader time hard codes an index of the shader time buffer within the
gen program.

In order to support shader time in the disk shader cache, we'd need to
add the shader time index into the program key. This should work, but
probably is not worth it for this particular debug feature.

Therefore, let's just disable the disk shader cache if the shader time
debug feature is used.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=106382


Fixes: 96fe36f7 "i965: Enable disk shader cache by default"
Cc: Eero Tamminen <eero.t.tamminen@intel.com>
Cc: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
(cherry picked from commit 3887700d)

Merged conflicts resolved by Dylan

Conflicts:
	src/mesa/drivers/dri/i965/brw_disk_cache.c