drm/amd/display: Allow DP register double buffer

Remove setting DP_DB_DISABLE to avoid issues when changing
bit depth after vbios take over.
Refactor code to perform single register update for both
pixel encoding and component depth fields.

Signed-off-by: Eric Bernstein <eric.bernstein@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>