drm/amd/display: Add const to a function

When calling the dc_link_bw_kbps_from_raw_frl_link_rate_data from
amdgpu_dm, GCC highlighted the following issue:

  In function ‘get_conv_frl_bw’:
  error: passing argument 1 of
  ‘dc_link_bw_kbps_from_raw_frl_link_rate_data’ discards ‘const’ qualifier
  from pointer target type [-Werror=discarded-qualifiers]
  997 |                                 aconnector->dc_link->dc,

This commit fix this issue by making DC a constant variable since we
only use it to invoke the function
bw_kbps_from_raw_frl_link_rate_data().
Reviewed-by: Chris Park <Chris.Park@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/core/dc_link_exports.c

@@ -285,8 +285,7 @@ int dc_link_aux_transfer_raw(struct ddc_service *ddc,
 			ddc, payload, operation_result);
 }
 
-uint32_t dc_link_bw_kbps_from_raw_frl_link_rate_data(
-		struct dc *dc, uint8_t bw)
+uint32_t dc_link_bw_kbps_from_raw_frl_link_rate_data(const struct dc *dc, uint8_t bw)
 {
 	return dc->link_srv->bw_kbps_from_raw_frl_link_rate_data(bw);
 }

drivers/gpu/drm/amd/display/dc/dc.h

@@ -1768,8 +1768,7 @@ void dc_restore_link_res_map(const struct dc *dc, uint32_t *map);
 bool dc_link_update_dsc_config(struct pipe_ctx *pipe_ctx);
 
 /* translate a raw link rate data to bandwidth in kbps */
-uint32_t dc_link_bw_kbps_from_raw_frl_link_rate_data(
-		struct dc *dc, uint8_t bw);
+uint32_t dc_link_bw_kbps_from_raw_frl_link_rate_data(const struct dc *dc, uint8_t bw);
 
 /* determine the optimal bandwidth given link and required bw.
  * @link - current detected link