drm/i915: drop cast from DEFINE_RES_MEM() usage

Since commit 52c4d11f ("resource: Convert DEFINE_RES_NAMED() to be
compound literal") it's no longer necessary to cast DEFINE_RES_MEM() to
struct resource.

This also fixes sparse warnings "cast from non-scalar" and "cast to
non-scalar".

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230116173422.1858527-2-jani.nikula@intel.com