drm/i915: check memory is mappable in read_from_page

On small-bar systems we could be given something non-mappable here,
which leads to nasty oops. Make this nicer by checking if the resource
is mappable or not, and return an error otherwise.

v2: drop GEM_BUG_ON(flags & I915_BO_ALLOC_GPU_ONLY)

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Jianshui Yu <jianshui.yu@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221004131916.233474-5-matthew.auld@intel.com