drm/qxl: remove unused `count` variable from `qxl_surface_id_alloc()`

drm/qxl: remove unused `count` variable from `qxl_surface_id_alloc()`

Clang 14 in an (essentially) defconfig loongarch64 build for next-20240326
reports [1]:

    drivers/gpu/drm/qxl/qxl_cmd.c:424:6: error: variable 'count' set
    but not used [-Werror,-Wunused-but-set-variable]

The variable is already unused in the version that got into the tree.

Thus remove the unused variable.

Fixes: f64122c1f6ad ("drm: add new QXL driver. (v1.4)")
Closes: https://lore.kernel.org/lkml/CANiq72mjc5t4n25SQvYSrOEhxxpXYPZ4pPzneSJHEnc3qApu2Q@mail.gmail.com/ [1]
Closes: https://lore.kernel.org/all/20240327163331.GB1153323@dev-arch.thelio-3990X/
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Link: https://lore.kernel.org/r/20240327175556.233126-1-ojeda@kernel.org
Signed-off-by: Maxime Ripard <mripard@kernel.org>