This patch to fix the gdm3 start failure with virual display:

/usr/libexec/gdm-x-session[1711]: (II) AMDGPU(0): Setting screen physical size to 270 x 203
/usr/libexec/gdm-x-session[1711]: (EE) AMDGPU(0): Failed to make import prime FD as pixmap: 22
/usr/libexec/gdm-x-session[1711]: (EE) AMDGPU(0): failed to set mode: Invalid argument
/usr/libexec/gdm-x-session[1711]: (WW) AMDGPU(0): Failed to set mode on CRTC 0
/usr/libexec/gdm-x-session[1711]: (EE) AMDGPU(0): Failed to enable any CRTC
gnome-shell[1840]: Running GNOME Shell (using mutter 42.2) as a X11 window and compositing manager
/usr/libexec/gdm-x-session[1711]: (EE) AMDGPU(0): failed to set mode: Invalid argument

vkms doesn't have modifiers support, set fb_modifiers_not_supported to bring the gdm back.
Signed-off-by: Yifan Zhang <yifan1.zhang@amd.com>
Acked-by: Guchun Chen <guchun.chen@amd.com>
Reviewed-by: Tim Huang <Tim.Huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>