drm/amd/display: Replace assert with error message in dp_retrieve_lttpr_cap()

[Why]
When assert in dp_retrieve_lttpr_cap() is hit, dmesg has traces like:

 RIP: 0010:dp_retrieve_lttpr_cap+0xcc/0x1a0 [amdgpu]
 Call Trace:
 <TASK>
  dp_retrieve_lttpr_cap+0xcc/0x1a0 [amdgpu]
  report_bug+0x1e8/0x240
  handle_bug+0x46/0x80
  link_detect+0x35/0x580 [amdgpu]

It happens when LTTPRs fail to increment dpcd repeater count.
We have a recovery action in place for such cases.
Assert is misleading, an indicative error in dmesg is more useful.

[How]
Remove ASSERT and use DC_LOG_ERROR instead.

Reviewed-by: Michael Strauss <michael.strauss@amd.com>
Signed-off-by: Jerry Zuo <jerry.zuo@amd.com>
Signed-off-by: Roman Li <roman.li@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>