intel/blorp: Fix possible NULL pointer dereferencing

Fix incomplete check of input params in blorp_surf_convert_to_uncompressed()
which can lead to NULL pointer dereferencing.

Fixes: 5ae8043f ("intel/blorp: Add an entrypoint for doing
bit-for-bit copies")
Fixes: f395d0ab ("intel/blorp: Internally expose
surf_convert_to_uncompressed")
Reviewed-by: Emil Velikov <emli.velikov@collabora.com>
Reviewed-by: Andres Gomez <agomez@igalia.com>
(cherry picked from commit cdb3eb71)
[Emil Velikov: drop non-applicable x/y hunk]
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>

Conflicts:
	src/intel/blorp/blorp_blit.c