drm/amd/display: Fix writeback job lock evasion within dm_crtc_high_irq

[Why]
Coverity report LOCK_EVASION warning. Access
acrtc->wb_pending without lock wb_conn->job_lock.

[How]
Lock wb_conn->job_lock before accessing
acrtc->wb_pending.

Reviewed-by: Alex Hung <alex.hung@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Hersen Wu <hersenxs.wu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>