drm/i915: Populate logical context during first pin.

This allows us to remove pin_map from state allocation, which saves
us a few retry loops. We won't need this until first pin, anyway.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20201231170405.22843-1-chris@chris-wilson.co.uk