drm/amd/display: use ARRAY_SIZE for base60_refresh_rates

Use ARRAY_SIZE instead of dividing sizeof array with sizeof an
element.

Clean up the following coccicheck warning:

./drivers/gpu/drm/amd/display/dc/core/dc_resource.c:448:47-48: WARNING:
Use ARRAY_SIZE.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>