drm/amd/display: pass panel instance in DMUB dirty rect command

[Why]

In cases where there are multiple eDP instances, DMUB needs to know
which instance the command is for. Today, the field for specifying the
panel_inst exists in both dmub_cmd_update_dirty_rect_data and
dmub_cmd_update_cursor_info_data.

For cursor updates, we already specify the panel_inst, but that's not
the case for dirty_rect updates. Today, a value of '0' is used (due
to initial memsetting of the cmd struct to 0)

[how]

In dc_dmub_update_dirty_rect(), Call dc_get_edp_link_panel_inst() to get
the panel_inst, and fill it in the DMUB cmd struct.

v2: Update commit message for clarity.

Signed-off-by: Mikita Lipski <mikita.lipski@amd.com>
Signed-off-by: David Zhang <dingchen.zhang@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>