drivers/gpu/drm/amd/amdgpu/gfx_v8_0.c: Remove duplicate header

Remove gca/gfx_8_0_enum.h which is included more than once

Signed-off-by: Brajeswar Ghosh <brajeswar.linux@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>