drm/i915: Check for all subplatform bits

Current code is checking only 2 bits in the subplatform, but actually 3
bits are allocated for the field. Check all 3 bits.

Fixes: 805446c8

 ("drm/i915: Introduce concept of a sub-platform")
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20210121161936.746591-1-tvrtko.ursulin@linux.intel.com


(cherry picked from commit 27b695ee1af9bb36605e67055874ec081306ac28)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>