drm/amd/display: Improve logic for is_lttpr_present

[WHY]
DP specifies that an LTTPR device is only present if PHY_REPEATER_CNT is
0x80, 0x40, 0x20, 0x10, 0x08, 0x04, 0x02, or 0x01.

All other values should be considered no LTTPRs present.

[HOW]
Function dp_convert_to_count already does this check. Use it to determine
if PHY_REPEATER_CNT is a valid LTTPR count.

Signed-off-by: Wesley Chalmers <Wesley.Chalmers@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Anson Jacob <Anson.Jacob@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>