drm/amd/display: remove unused average_render_time_in_us and i variables

clang with W=1 reports
drivers/gpu/drm/amd/amdgpu/../display/modules/freesync/freesync.c:1132:15: error: variable
  'average_render_time_in_us' set but not used [-Werror,-Wunused-but-set-variable]
        unsigned int average_render_time_in_us = 0;
                     ^
This variable is not used so remove it, which caused i to be unused so remove that as well.

Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>