drm/amd/pm: re-enable MC access after PrepareMp1ForUnload on SMU V15 APUs

drm/amd/pm: re-enable MC access after PrepareMp1ForUnload on SMU V15 APUs

During smu_v15_0_0_system_features_control(), the driver sends a
PrepareMp1ForUnload message to PMFW. PMFW then performs nBIF and SYSHUB
function-level resets (FLR), disabling PCIe CFG space reset, which
clears the framebuffer enable bit to zero and disables MC (memory controller)
access from the host.

Re-enable MC access via the nbio mc_access_enable callback right after
PrepareMp1ForUnload completes in smu_v15_0_0_system_features_control().

Signed-off-by: Shubhankar Milind Sardeshpande <Shubhankar.MilindSardeshpande@amd.com>
Signed-off-by: Suresh Guttula <Suresh.Guttula@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 840a3c5aeae779a3bc75d7f747c3ed18b1af6507)
Cc: stable@vger.kernel.org