drm/amdgpu/disply: set num_crtc earlier

To avoid a recently added warning:
 Bogus possible_crtcs: [ENCODER:65:TMDS-65] possible_crtcs=0xf (full crtc mask=0x7)
 WARNING: CPU: 3 PID: 439 at drivers/gpu/drm/drm_mode_config.c:617 drm_mode_config_validate+0x178/0x200 [drm]
In this case the warning is harmless, but confusing to users.

Bug: https://bugzilla.kernel.org/show_bug.cgi?id=209123


Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>