i965,iris/blorp: do not blit 0-sizes

i965,iris/blorp: do not blit 0-sizes

Seems there is no sense in blitting 0-sized sources
or destinations.
Additionaly it may cause segfaults for i965.

v2: Function call replaced with inline check

v3: Added check to avoid devision by zero (L. Landwerlin)

v4: Added simillar check for Iris (L. Landwerlin)

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=110239
Signed-off-by: Sergii Romantsov <sergii.romantsov@globallogic.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>