drm/amdgpu: fix compiler warning(v2)

warning: ISO C90 forbids mixed declarations and code [-Wdeclaration-after-statement]
  int write = !(gtt->userflags & AMDGPU_GEM_USERPTR_READONLY);

v2: put short variable declaration last

Signed-off-by: Guchun Chen <guchun.chen@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>