drm/amd/display: avoid stringop-overflow warnings for dp_decide_lane_settings()

gcc prints a warning about a possible array overflow for a couple of
callers of dp_decide_lane_settings() after commit 1b56c90018f0 ("Makefile:
Enable -Wstringop-overflow globally"):

drivers/gpu/drm/amd/amdgpu/../display/dc/link/protocols/link_dp_training_fixed_vs_pe_retimer.c: In function 'dp_perform_fixed_vs_pe_training_sequence_legacy':
drivers/gpu/drm/amd/amdgpu/../display/dc/link/protocols/link_dp_training_fixed_vs_pe_retimer.c:426:25: error: 'dp_decide_lane_settings' accessing 4 bytes in a region of size 1 [-Werror=stringop-overflow=]
  426 |                         dp_decide_lane_settings(lt_settings, dpcd_lane_adjust,
      |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  427 |                                         lt_settings->hw_lane_settings, lt_settings->dpcd_lane_settings);
      |                                         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/gpu/drm/amd/amdgpu/../display/dc/link/protocols/link_dp_training_fixed_vs_pe_retimer.c:426:25: note: referencing argument 4 of type 'union dpcd_training_lane[4]'

I'm not entirely sure what caused this, but changing the prototype to expect
a pointer instead of an array avoids the warnings.

Fixes: 7727e7b6 ("drm/amd/display: Improve robustness of FIXED_VS link training at DP1 rates")
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>