docs: amdgpu: fix a warning when building the documentation

As reported by Sphinx:

	Documentation/gpu/amdgpu.rst:200: WARNING: Inline emphasis start-string without end-string.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>