drm/amd/display: Default to linear output gamma

[Why] Our output TF calculation doesn't work if no user-specified gamma
correction. Normally, user provides this, but driver sohuld just assume
default (linear) gamma otherwise.

[How] Remove output TF dependency on user gamma being provided.

Signed-off-by: Krunoslav Kovac <Krunoslav.Kovac@amd.com>
Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Acked-by: Sivapiriyan Kumarasamy <Sivapiriyan.Kumarasamy@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>