Documentation/gpu: Document how to narrow down display issues

The amdgpu driver is composed of multiple components, each of which can
be a source of some specific problem that the user/developer can see.
This commit introduces steps to narrow down and collect display
information.

Cc: Leo Li <sunpeng.li@amd.com>
Cc: Aurabindo Pillai <aurabindo.pillai@amd.com>
Cc: Hamza Mahfooz <hamza.mahfooz@amd.com>
Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Mario Limonciello <mario.limonciello@amd.com>
Cc: Christian Konig <christian.koenig@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>