xfree86: Detect the primary device by checking outputs

Before this patch, X server detects/sets the primary device by:
1. The "PrimaryGPU" option in extra X configuration
2. pci_device_is_boot_vga() for PCI devices
3. Set the first (0 index) device as the primary device, if it is not
   found yet.

However, the other display controllers like Amlogic's meson cannot be
detected as the primary device by pci_device_is_boot_vga(). Thus, it has
to set the extra X configuration for the "PrimaryGPU" option.
Otherwise, X server will set the first (0 index) device as the primary
device. But it may not be the correct one, because it has no output.
For example, Amlogic puts the GPU and display controller as different
devices:

(II) xfree86: Adding drm device (/dev/dri/card0)
(II) Platform probe for /sys/devices/platform/soc/d0000000.apb/d00c0000.gpu/drm/card0
(II) xfree86: Adding drm device (/dev/dri/card1)
(II) Platform probe for /sys/devices/platform/soc/d0100000.vpu/drm/card1

This patch adds a new way to set the primary device by detecting the
output connecters of devices in platform dev driver, before use the
first platform device as a fallback. Get the amount of the device's
connectors by referring to the check_outputs() function in modesetting
driver.

Buglink: xorg/xserver#1023
Signed-off-by: Jian-Hong Pan <jian-hong@endlessm.com>
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