drm/amdgpu: enable gfx eop interrupt per gfx pipe

Navi10 has 2 gfx pipe and need to enable gfx eop interrupt
per pipe, instead of enable eop int for all gfx pipes at one
time.

Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>