drm/amdgpu/gfx: use proper offset define for MEC doorbells

Looks like a copy paste typo.

Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
This commit is contained in:
Alex Deucher 2018-11-16 14:46:51 -05:00
commit 5814cef807