drm/amd/display: clean up assignment of amdgpu_crtc

The declaration of pointer amdgpu_crtc has a redundant assignment to
amdgpu_crtc. Clean this up by removing it.

Detected by CoverityScan, CID#1460299 ("Evaluation order violation")

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>