drm/amdgpu: Enable gfxoff quirk on MacBook Pro

Enabling gfxoff quirk results in perfectly usable graphical user
interface on MacBook Pro (15-inch, 2019) with Radeon Pro Vega 20 4 GB.

Without the quirk, X server is completely unusable as every few seconds
there is gpu reset due to ring gfx timeout.

Signed-off-by: Tomasz Moń <desowin@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org