drm/amdgpu/gfx_v12_0: set gfx.rs64_enable from PFP header on GFX12

drm/amdgpu/gfx_v12_0: set gfx.rs64_enable from PFP header on GFX12

gfx_v12_0_init_microcode() always loads RS64 CP ucode but never set
adev->gfx.rs64_enable, so it stayed false and code that branches on it
(e.g. MEC pipe reset) used the legacy CP_MEC_CNTL path incorrectly.

Match GFX11: derive RS64 mode from the PFP firmware header (v2.0) via
amdgpu_ucode_hdr_version(). Log at debug when RS64 is enabled.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Jesse Zhang <jesse.zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>