drm/i915: pass dev_priv explicitly to DSPOFFSET

Avoid the implicit dev_priv local variable use, and pass dev_priv
explicitly to the DSPOFFSET register macro.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/c1d487d2c753221144e8fb8f17e5eb2826dba5f2.1716469091.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>