linuxkpi: Support s2idle in pm_suspend_target_state

When going to s2idle, set Linux' pm_suspend_target_state to
PM_SUSPEND_TO_IDLE instead of setting PM_SUSPEND_MEM (for S3) in all
cases.

This is necessary because more recent versions of the amdgpu DRM drivers
have different paths for when entering s2idle & S0ix is supported than
S3.

Reviewed by: bz
Approved by: bz
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D51591