drm/amdgpu: Ensure ret is always initialized when using SOC15_WAIT_ON_RREG

drm/amdgpu: Ensure ret is always initialized when using SOC15_WAIT_ON_RREG

Commit b0f3cd3191cd ("drm/amdgpu: remove unnecessary JPEG2.0 code from
VCN2.0") introduced a new clang warning in the vcn_v2_0_stop function:

../drivers/gpu/drm/amd/amdgpu/vcn_v2_0.c:1082:2: warning: variable 'r'
is used uninitialized whenever 'while' loop exits because its condition
is false [-Wsometimes-uninitialized]
        SOC15_WAIT_ON_RREG(VCN, 0, mmUVD_STATUS, UVD_STATUS__IDLE, 0x7, r);
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../drivers/gpu/drm/amd/amdgpu/../amdgpu/soc15_common.h:55:10: note:
expanded from macro 'SOC15_WAIT_ON_RREG'
                while ((tmp_ & (mask)) != (expected_value)) {   \
                       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../drivers/gpu/drm/amd/amdgpu/vcn_v2_0.c:1083:6: note: uninitialized use
occurs here
        if (r)
            ^
../drivers/gpu/drm/amd/amdgpu/vcn_v2_0.c:1082:2: note: remove the
condition if it is always true
        SOC15_WAIT_ON_RREG(VCN, 0, mmUVD_STATUS, UVD_STATUS__IDLE, 0x7, r);
        ^
../drivers/gpu/drm/amd/amdgpu/../amdgpu/soc15_common.h:55:10: note:
expanded from macro 'SOC15_WAIT_ON_RREG'
                while ((tmp_ & (mask)) != (expected_value)) {   \
                       ^
../drivers/gpu/drm/amd/amdgpu/vcn_v2_0.c:1072:7: note: initialize the
variable 'r' to silence this warning
        int r;
             ^
              = 0
1 warning generated.

To prevent warnings like this from happening in the future, make the
SOC15_WAIT_ON_RREG macro initialize its ret variable before the while
loop that can time out. This macro's return value is always checked so
it should set ret in both the success and fail path.

Link: https://github.com/ClangBuiltLinux/linux/issues/776
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>