anv: ignore VkSamplerYcbcrConversion on non-yuv formats

anv: ignore VkSamplerYcbcrConversion on non-yuv formats

This fulfills a requirement for clients that want to utilize same
code path for images with external formats (VK_FORMAT_UNDEFINED) and
"regular" RGBA images where format is known. This is similar to how
OES_EGL_image_external works.

To support this, we allow color conversion samplers for non-YUV
formats but skip setting up conversion when format does not have
can_ycbcr flag set.

v2: add comment and bundle can_ycbcr to the existing break
    condition (Lionel)

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>