agp/intel: Fix a memory leak on module initialisation failure

In intel_gtt_setup_scratch_page(), pointer "page" is not released if
pci_dma_mapping_error() return an error, leading to a memory leak on
module initialisation failure.  Simply fix this issue by freeing "page"
before return.

Fixes: 0e87d2b0

 ("intel-gtt: initialize our own scratch page")
Signed-off-by: Qiushi Wu <wu000273@umn.edu>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20200522083451.7448-1-chris@chris-wilson.co.uk