drm/amdgpu: Fix mc_umc_status used uninitialized warning

drm/amdgpu: Fix mc_umc_status used uninitialized warning

On ChromeOS clang build, the following warning is seen:

/mnt/host/source/src/third_party/kernel/v5.15/drivers/gpu/drm/amd/amdgpu/umc_v6_7.c:463:6: error: variable 'mc_umc_status' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]
        if (mca_addr == UMC_INVALID_ADDR) {
            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/mnt/host/source/src/third_party/kernel/v5.15/drivers/gpu/drm/amd/amdgpu/umc_v6_7.c:485:21: note: uninitialized use occurs here
        if ((REG_GET_FIELD(mc_umc_status, MCA_UMC_UMC0_MCUMC_STATUST0, Val) == 1 &&
                           ^~~~~~~~~~~~~
/mnt/host/source/src/third_party/kernel/v5.15/drivers/gpu/drm/amd/amdgpu/../amdgpu/amdgpu.h:1208:5: note: expanded from macro 'REG_GET_FIELD'
        (((value) & REG_FIELD_MASK(reg, field)) >> REG_FIELD_SHIFT(reg, field))
           ^~~~~
/mnt/host/source/src/third_party/kernel/v5.15/drivers/gpu/drm/amd/amdgpu/umc_v6_7.c:463:2: note: remove the 'if' if its condition is always true
        if (mca_addr == UMC_INVALID_ADDR) {
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/mnt/host/source/src/third_party/kernel/v5.15/drivers/gpu/drm/amd/amdgpu/umc_v6_7.c:460:24: note: initialize the variable 'mc_umc_status' to silence this warning
        uint64_t mc_umc_status, mc_umc_addrt0;
                              ^
                               = 0
1 error generated.
make[5]: *** [/mnt/host/source/src/third_party/kernel/v5.15/scripts/Makefile.build:289: drivers/gpu/drm/amd/amdgpu/umc_v6_7.o] Error 1

Fix by initializing mc_umc_status = 0.

Fixes: 1014bd1cb32552 ("drm/amdgpu: support to convert dedicated umc mca address")
Reviewed-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>