drm/amd/display: Avoid reading LTTPR caps in non-LTTPR mode.

[Why]
LTTPR capable devices on the DisplayPort path may assume that
extended LTTPR AUX timeouts will be used after LTTPR capabilities
are read.

When DPTX operates in non-LTTPR mode, AUX timeouts are not
extended and this can result in AUX transactions timing out.

[How]
Use shared helper function to determine LTTPR mode and do not
read LTTPR capabilities in non-LTTPR mode.

Reviewed-by: Mustapha Ghaddar <Mustapha.Ghaddar@amd.com>
Reviewed-by: Meenakshikumar Somasundaram <Meenakshikumar.Somasundaram@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Jimmy Kizito <Jimmy.Kizito@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>