drm/amd/display: Reduce accessing remote DPCD overhead

commit adb4998f4928a17d91be054218a902ba9f8c1f93 upstream.

[Why]
Observed frame rate get dropped by tool like glxgear. Even though the
output to monitor is 60Hz, the rendered frame rate drops to 30Hz lower.

It's due to code path in some cases will trigger
dm_dp_mst_is_port_support_mode() to read out remote Link status to
assess the available bandwidth for dsc maniplation. Overhead of keep
reading remote DPCD is considerable.

[How]
Store the remote link BW in mst_local_bw and use end-to-end full_pbn
as an indicator to decide whether update the remote link bw or not.

Whenever we need the info to assess the BW, visit the stored one first.

Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/3720
Fixes: fa57924c76d9 ("drm/amd/display: Refactor function dm_dp_mst_is_port_support_mode()")
Cc: Mario Limonciello <mario.limonciello@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Jerry Zuo <jerry.zuo@amd.com>
Signed-off-by: Wayne Lin <Wayne.Lin@amd.com>
Signed-off-by: Tom Chung <chiahsuan.chung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 4a9a918545455a5979c6232fcf61ed3d8f0db3ae)
Cc: stable@vger.kernel.org
Signed-off-by: Wayne Lin <Wayne.Lin@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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