drm/i915: Make I915_GEM_IDLE_TIMEOUT into a macro

Currently we use HZ/5 for detecting a dead gpu on startup, and we will
wish to reuse this value for detecting a dead gpu on suspend, so convert
it into a macro for later convenience.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190307104530.21745-1-chris@chris-wilson.co.uk