drm/amd/display: Spinlock before reading event

[WHY & HOW]
A read of acrtc_attach->base.state->event was not locked so moving it
inside the spinlock.

This fixes a LOCK_EVASION issue reported by Coverity.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>