mesa: Fix error code generation in glReadPixels()

mesa: Fix error code generation in glReadPixels()

Section 4.3.1, page 220, of OpenGL 3.3 specification explains
the error conditions for glreadPixels():

   "If the format is DEPTH_STENCIL, then values are taken from
    both the depth buffer and the stencil buffer. If there is
    no depth buffer or if there is no stencil buffer, then the
    error INVALID_OPERATION occurs. If the type parameter is
    not UNSIGNED_INT_24_8 or FLOAT_32_UNSIGNED_INT_24_8_REV,
    then the error INVALID_ENUM occurs."

Fixes failing Khronos CTS test packed_depth_stencil_error.test

V2: Avoid code duplication

Cc: <mesa-stable@lists.freedesktop.org>
Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>