drm/amd/display: Fix DTBCLK disable requests and SRC_SEL programming

[Description]
- When transitioning FRL / DP2 is not required, we will always request
  DTBCLK = 0Mhz, but PMFW returns the min freq
- This causes us to make DTBCLK requests every time we call optimize
  after transitioning from FRL to non-FRL
- If DTBCLK is not required, request the min instead (then we only need
  to make 1 extra request at boot time)
- Also when programming PIPE_DTO_SRC_SEL, don't programming for DP
  first, just programming once for the required selection (programming
  DP on an HDMI connection then switching back causes corruption)

Reviewed-by: Dillon Varone <Dillon.Varone@amd.com>
Acked-by: Jasdeep Dhillon <jdhillon@amd.com>
Signed-off-by: Alvin Lee <Alvin.Lee2@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>