intel: Suppress a compiler warning for an pointer->int cast

intel: Suppress a compiler warning for an pointer->int cast

intel_pixel_read.c: In function ‘do_blit_readpixels’:
intel_pixel_read.c:221: warning: cast from pointer to integer of
different size

Cast via an intermediate (GLintptr) instead and hope the result fits
within GLuint... [It should as we simply do not support textures *that*
large!]

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>