drm/i915: Use ERR_CAST instead of ERR_PTR(PTR_ERR())

Fix following coccicheck warning:
./drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c:3117:15-22: WARNING:
ERR_CAST can be used with eb->requests[i].

Signed-off-by: Wan Jiabing <wanjiabing@vivo.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211025113316.24424-1-wanjiabing@vivo.com