drm/amd/display: Increase DPCD read retries

[Why]
Empirical measurement of some monitors that fail to read EDID while
booting shows that the number of retries with a 30ms delay between
tries is as high as 16.

[How]
Increase number of retries to 20.

Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/4672
Reviewed-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Mario Limonciello (AMD) <superm1@kernel.org>
Signed-off-by: Ivan Lipski <ivan.lipski@amd.com>
Tested-by: Dan Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit ad1c59ad7cf74ec06e32fe2c330ac1e957222288)
Cc: stable@vger.kernel.org