hwcontext_drm: make dependency on Linux kernel headers optional

hwcontext_drm: make dependency on Linux kernel headers optional

This was introduced in version 4.6. And may not exist all without an
optional package. So to prevent a hard dependency on needing the Linux
kernel headers to compile, make this optional.

Also ignore the status of the ioctl, since it may fail on older kernels
which don't support it. It's okay to ignore as its not fatal and any
serious errors will be caught later by the mmap call.