drm/amdgpu/gfx7: fix function name

Was using the wrong prefix (gmc rather than gfx).  The function
is related to the gfx hw, not gmc.  This also makes it consistent
with the naming in gfx8.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>