drm/i915/psr: fix copy-paste error with setting of tp2_wakeup_time_us

Currently for the psr_table->tp2_tp3_wakeup_time case 3 there appears
to be a copy-paste error from the previous switch statement where
dev_priv->vbt.psr.tp1_wakeup_time_us is being assigned and I believe
it should be dev_priv->vbt.psr.tp2_tp3_wakeup_time_us that should be
assigned instead.

Detected by CoverityScan, CID#1470105 ("Copy-paste error")

Fixes: 77312ae8 ("drm/i915/psr: vbt change for psr")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180620132543.28092-1-colin.king@canonical.com