drm/amd/display: Avoid enum conversion warning

Clang warns (or errors with CONFIG_WERROR=y) when performing arithmetic
with different enumerated types, which is usually a bug:

    drivers/gpu/drm/amd/amdgpu/../display/dc/link/protocols/link_dp_dpia_bw.c:548:24: error: arithmetic between different enumeration types ('const enum dc_link_rate' and 'const enum dc_lane_count') [-Werror,-Wenum-enum-conversion]
      548 |                         link_cap->link_rate * link_cap->lane_count * LINK_RATE_REF_FREQ_IN_KHZ * 8;
          |                         ~~~~~~~~~~~~~~~~~~~ ^ ~~~~~~~~~~~~~~~~~~~~
    1 error generated.

In this case, there is not a problem because the enumerated types are
basically treated as '#define' values. Add an explicit cast to an
integral type to silence the warning.

Closes: https://github.com/ClangBuiltLinux/linux/issues/1976
Fixes: 5f3bce13 ("drm/amd/display: Request usb4 bw for mst streams")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/link/protocols/link_dp_dpia_bw.c

@@ -544,8 +544,9 @@ int link_dp_dpia_get_dp_overhead_in_dp_tunneling(struct dc_link *link)
 		 */
 		const struct dc_link_settings *link_cap =
 			dc_link_get_link_cap(link);
-		uint32_t link_bw_in_kbps =
-			link_cap->link_rate * link_cap->lane_count * LINK_RATE_REF_FREQ_IN_KHZ * 8;
+		uint32_t link_bw_in_kbps = (uint32_t)link_cap->link_rate *
+					   (uint32_t)link_cap->lane_count *
+					   LINK_RATE_REF_FREQ_IN_KHZ * 8;
 		link_mst_overhead = (link_bw_in_kbps / 64) + ((link_bw_in_kbps % 64) ? 1 : 0);
 	}