drm/amd/display: Remove redundant DRM device struct in amdgpu_dm_, mst_types.c

Declaration of 'struct drm_device *dev' is redundant, as
'connector->dev' & 'dev_get_drvdata(kdev)' can be directly passed to
'drm_to_adev', without any intermediate DRM device 'dev' variable

Cc: Roman Li <roman.li@amd.com>
Cc: Hamza Mahfooz <hamza.mahfooz@amd.com>
Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Cc: Aurabindo Pillai <aurabindo.pillai@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Roman Li <roman.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>