drm/amd/display: Fix && vs || typos

These ANDs should be ORs or it will lead to a NULL dereference.

Fixes: fb5a3d03 ("drm/amd/display: Add NULL test for 'timing generator' in 'dcn21_set_pipe()'")
Fixes: 886571d2 ("drm/amd/display: Fix 'panel_cntl' could be null in 'dcn21_set_backlight_level()'")
Reviewed-by: Anthony Koo <anthony.koo@amd.com>
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>