drm/i915/selftest: Fix ktime_get() and h/w access order

Use ktime_get() after accessing the mmio or any driver resource,
while using wall time for various calculation that depends on
the inserted delay in order to account any mmio and resource
access latency.

Cc: Chris Wilson <chris.p.wilson@intel.com>
Signed-off-by: Anshuman Gupta <anshuman.gupta@intel.com>
Reviewed-by: Badal Nilawar <badal.nilawar@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230223100503.3323627-3-anshuman.gupta@intel.com