drm/i915: Pass size to oprom_get_vbt

oprom_get_vbt will later be used to show the contents of vbt for which
the size of vbt is needed.

v2: Avoid overuse of *size and remove dummy size variable in
    intel_bios_init(Jani)

Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240228213235.2495611-2-radhakrishna.sripada@intel.com