drm/amd/display: remove set but not used variable 'min_content'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/gpu/drm/amd/display/modules/color/color_gamma.c: In function build_freesync_hdr:
drivers/gpu/drm/amd/display/modules/color/color_gamma.c:830:20: warning: variable min_content set but not used [-Wunused-but-set-variable]

It is not used since commit 50575eb5 ("drm/amd/display:
Only use EETF when maxCL > max display")

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: zhengbin <zhengbin13@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>