drm/amd/amdgpu: Fix build errors due to declarations after labels

drm/amd/amdgpu: Fix build errors due to declarations after labels

In C90 (which the kernel uses with -std=gnu89), declarations must
appear at the beginning of a block and cannot follow a label. The
switch cases in amdgpu_discovery.c and gmc_v12_1.c contained variable
declarations immediately after case labels, causing the compiler to
error:

drivers/gpu/drm/amd/amdgpu/gmc_v12_1.c:533:3: error: a label can only be
part of a statement and a declaration is not a statement

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>