drm/amd: fix gfx hang on renoir in IGT reload test

[why]
CP hangs in igt reloading test on renoir, more precisely, hangs on the
second time insmod.

[how]
mode2 reset can make it recover, and mode2 reset only effects gfx core,
dcn and the screen will not be impacted.

Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Tianci.Yin <tianci.yin@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>