drm/i915/fbdev: lock the fbdev obj before vma pin

lock the fbdev obj before calling into
i915_vma_pin_iomap(). This helps to solve below :

<7>[   93.563308] i915 0000:00:02.0: [drm:intelfb_create [i915]] no BIOS fb, allocating a new one
<4>[   93.581844] ------------[ cut here ]------------
<4>[   93.581855] WARNING: CPU: 12 PID: 625 at drivers/gpu/drm/i915/gem/i915_gem_pages.c:424 i915_gem_object_pin_map+0x152/0x1c0 [i915]

Fixes: f0b6b01b ("drm/i915: Add ww context to intel_dpt_pin, v2.")
Cc: Chris Wilson <chris.p.wilson@intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Tejas Upadhyay <tejas.upadhyay@intel.com>
Signed-off-by: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230301201053.928709-5-radhakrishna.sripada@intel.com