drm/amdgpu: fix error checking in amdgpu_read_mm_registers for soc15

drm/amdgpu: fix error checking in amdgpu_read_mm_registers for soc15

BugLink: https://bugs.launchpad.net/bugs/2016875
commit 0dcdf8498eae2727bb33cef3576991dc841d4343 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: Paolo Pisati <paolo.pisati@canonical.com>