drm/v3d: Add clock handling

Since the initial commit 57692c94 ("drm/v3d: Introduce a new DRM driver
for Broadcom V3D V3.x+") the struct v3d_dev reserved a pointer for
an optional V3D clock. But there wasn't any code, which fetched it.
So add the missing clock handling before accessing any V3D registers.

Signed-off-by: Stefan Wahren <wahrenst@gmx.net>
Reviewed-by: Maíra Canal <mcanal@igalia.com>
Signed-off-by: Maíra Canal <mcanal@igalia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250201125046.33030-1-wahrenst@gmx.net