xf86drm: continue after drmProcessPlatformDevice failure

On ChromeOS devices, readdir() processes the directory in
the following order:

-NAME-              -TYPE-
.                    n/a
..                   n/a
vgem                 n/a
card1           DRM_BUS_PLATFORM
renderD129      DRM_BUS_PLATFORM
card0             DRM_BUS_PCI
renderD128        DRM_BUS_PCI
controlD64        DRM_BUS_PCI

In drmGetDevices2, after drmProcessPlatformDevice fails for
/dev/dri/card1, we don't process the remaining directory entries.
As such, Vulkan fails to initialize since Mesa uses drmGetDevices2.
To fix this, continue if drmProcessPlatformDevice fails.

Fixes: 7b1f37f4 ("xf86drm: Add platform and host1x bus support")
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
[Emil: correct the host1x platforms as well]
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>