drm/amd/display: Add null checks before accessing struct elements

drm/amd/display: Add null checks before accessing struct elements

[WHAT]
1. is_pwrseq0 needs to check link before accessing link->link_index.
2. context is checked before accessing its bw_ctx.dml2
3. clk_mgr_base->bw_params is checked before clk_table.num_entries_per_cl

This fixes 4 REVERSE_INULL issues reported by Coverity.

Reviewed-by: Rodrigo Siqueira <rodrigo.siqueira@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>