drm/amd/display: Don't program bypass on linear regamma LUT

Even though this is required for degamma since DCE HW only supports a
couple predefined LUTs we can just program the LUT directly for regamma.

This fixes dark screens which occurs when we program regamma to bypass
while degamma is using srgb LUT.

Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Leo Li <sunpeng.li@amd.com>
Cc: stable@vger.kernel.org
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>