drm/xe/display: Avoid calling readq()

readq() is not available in 32bits and i915_gem_object_read_from_page()
is supposed to allow reading arbitrary sizes determined by the `size`
argument. Currently the only caller only passes a size == 8 so the
second problem is not that big. Migrate to calling
memcpy()/memcpy_fromio() to allow possible changes in the display side
and to fix the build on 32b architectures.

v2: Use memcpy/memcpy_fromio directly rather than using iosys-map with
    the same size == 8 bytes restriction (Matt Roper)

Fixes: 44e69495

 ("drm/xe/display: Implement display support")
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240119001612.2991381-4-lucas.demarchi@intel.com