i965: Fix is-renderable check in intel_image_target_renderbuffer_storage

i965: Fix is-renderable check in intel_image_target_renderbuffer_storage

Previously we could create a renderbuffer with format
MESA_FORMAT_R8G8B8A8_UNORM, convert that renderbuffer to an EGLImage,
then FAIL to convert the EGLImage back to a renderbuffer because
reasons.  Just use the same check in
intel_image_target_renderbuffer_storage that brw_render_target_supported
uses.

There are more checks in brw_render_target_supported, but I don't think
they are necessary here.  A different approach would be to refactor
brw_render_target_supported to take rb->Format and rb->NumSamples as
parameters (instead of a gl_renderbuffer) and use the new function here.

Fixes:

    ES2-CTS.gtf.GL2ExtensionTests.egl_image.egl_image

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>
Tested-by: Tapani Pälli <tapani.palli@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=92476
Cc: "10.3 10.4 10.5 10.6 11.0" <mesa-stable@lists.freedesktop.org>