Revert "i965: get PrimitiveMode from the program rather than the shader struct"

Revert "i965: get PrimitiveMode from the program rather than the shader struct"

This reverts commit 644e015f0b9236e955d679cac4bcc7a1523fc475.

PrimitiveMode from the program doesn't always hold a valid value that
is neither of GL_TRIANGLES, GL_QUADS nor GL_ISOLINES when reaching
this code. This caused regressions in the following CTS tests:
  GL44-CTS.stencil_texturing.functional
  GL44-CTS.shading_language_420pack.binding_images
  GL44-CTS.shading_language_420pack.binding_samplers
  GL44-CTS.shading_language_420pack.binding_uniform_single_block
  GL44-CTS.shading_language_420pack.implicit_conversions
  GL44-CTS.shading_language_420pack.initializer_list
  GL44-CTS.shading_language_420pack.length_of_vector_and_matrix
  GL44-CTS.shading_language_420pack.line_continuation

Hence, we rather take it from the linked shader.

Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>
Signed-off-by: Andres Gomez <agomez@igalia.com>