drm/i915: Decrease number of subplatform bits

drm/i915: Decrease number of subplatform bits

Commit 6ce1c33d6c36 ("drm/i915: Kill INTEL_SUBPLATFORM_AML") removed the
only platform which used bit 2 so could also decrease the
INTEL_SUBPLATFORM_BITS definition.

This is not a fixes material but still lets make it precise.

v2:
 * Fix assert in intel_device_info_subplatform_init by introducing
   INTEL_SUBPLATFORM_MASK. (Chris)
 * Update intel_subplatform().

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
References: 6ce1c33d6c36 ("drm/i915: Kill INTEL_SUBPLATFORM_AML")
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20210121161936.746591-2-tvrtko.ursulin@linux.intel.com
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>