devices: gpu: Check /dev/dri exists before mount
devices: gpu: Check /dev/dri exists before mount

GPU in 2D mode does not need this and /dev/dri may not
exist (Nvidia startup failure after forced workstation
updates?).

BUG=b:166180769
TEST=boot Cuttlefish

Change-Id: Iaf80fc0f8f031270267df18d61964f8a648920b4
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/2381892
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Gurchetan Singh <99434bba058e25b6203b8ae055a4f697dad0ef64@chromium.org>
Reviewed-by: Zach Reizner <3374927928a53d04457674c546bd4de57c2044ec@chromium.org>
Commit-Queue: Jason Macnak <76caa58940b8c6785e996f522e85b625ac7c7baa@google.com>

google/crosvm,google/crosvm,google/crosvm,google/crosvm