drm/amd/display: Check pipe offset before setting vblank

pipe_ctx has a size of MAX_PIPES so checking its index before accessing
the array.

This fixes an OVERRUN issue reported by Coverity.

Reviewed-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Acked-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>