loader: use a maximum of 64 drmDevices

loader: use a maximum of 64 drmDevices

Currently that's the hard-coded maximum in the kernel, even though the
libdrm API allows for more. Latter is done with extendability in mind.

Allocate 64 pointers^Wdevices on stack for now. Making for shorter and
ever-so-slightly faster code.

v2: Use single MAX_DRM_DEVICES #define (Eric)

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Adam Jackson <ajax@redhat.com> (v1)
Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4084>