drm/radeon: remove set but not used variable 'dig_connector'

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

drivers/gpu/drm/radeon/atombios_dp.c: In function radeon_dp_get_panel_mode:
drivers/gpu/drm/radeon/atombios_dp.c:415:36: warning: variable dig_connector set but not used [-Wunused-but-set-variable]

It is not used since commit 379dfc25 ("drm/radeon/dp:
switch to the common i2c over aux code")

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