amd/display/amdgpu_dm: delete same check in if condition

In function amdgpu_dm_connector_get_modes, drm_edid_is_valid
will check weather (!edid), no need to check again in the if
branch.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Bernard Zhao <bernard@vivo.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>