drm/tests: Alloc drm_device on drm_exec tests

The drm_exec tests where crashing[0] because of a null dereference. This
is caused by a new access of the `driver` attribute of `struct
drm_driver` on drm_gem_private_object_init(). Alloc the drm_device to
fix that.

[0]
[15:05:24] ================== drm_exec (6 subtests) ===================
[15:05:24] [PASSED] sanitycheck
^CERROR:root:Build interruption occurred. Cleaning console.
[15:05:50] [ERROR] Test: drm_exec: missing expected subtest!
[15:05:50] BUG: kernel NULL pointer dereference, address: 00000000000000b0
[15:05:50] #PF: supervisor read access in kernel mode
[15:05:50] #PF: error_code(0x0000) - not-present page
[15:05:50] PGD 0 P4D 0
[15:05:50] Oops: 0000 [#1] PREEMPT NOPTI
[15:05:50] CPU: 0 PID: 23 Comm: kunit_try_catch Tainted: G                 N 6.4.0-rc7-02032-ge6303f323b1a #69
[15:05:50] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.2-1.fc37 04/01/2014
[15:05:50] RIP: 0010:drm_gem_private_object_init+0x60/0xc0

Fixes: e6303f32 ("drm: manager to keep track of GPUs VA mappings")
Signed-off-by: Arthur Grillo <arthurgrillo@riseup.net>
Tested-by: Danilo Krummrich <dakr@redhat.com>
Acked-by: Danilo Krummrich <dakr@redhat.com>
Reviewed-by: Maíra Canal <mcanal@igalia.com>
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230731182241.240556-1-arthurgrillo@riseup.net