drm/i915: Make pre-production detection use direct revid comparison

Although we're converting our workarounds to use a revid->stepping
lookup table, the function that detects pre-production hardware should
continue to compare against PCI revision ID values directly.  These are
listed in the bspec as integers, so it's easier to confirm their
correctness if we just use an integer literal rather than a symbolic
name anyway.

Bspec: 13620, 19131, 13626, 18329
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Anusha Srivatsa <anusha.srivatsa@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210713193635.3390052-3-matthew.d.roper@intel.com