drm/amdgpu: fix error checking in amdgpu_read_mm_registers for soc15

stable inclusion
from stable-v5.10.175
commit 4279e87da6c88d63796904184b6f027321e370ab
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I8711T

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4279e87da6c88d63796904184b6f027321e370ab

--------------------------------

commit 0dcdf849 upstream.

Properly skip non-existent registers as well.

Bug: https://gitlab.freedesktop.org/drm/amd/-/issues/2442


Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: sanglipeng <sanglipeng1@jd.com>