drm/radeon: enable ABGR and XBGR formats (v2)

Add support for DRM_FORMAT_{A,X}BGR8888 in atombios_crtc
Swapping of red and blue channels is implemented for radeon chipsets:
DCE2/R6xx and later - crossbar registers defined where needed and used
DCE1/R5xx - AVIVO_D1GRPH_SWAP_RB bit is used

(v2) Set AVIVO_D1GRPH_SWAP_RB bit in fb_format, using bitwise OR for DCE1 path
     Use bitwise OR where required for big endian settings in fb_swap
     Use existing code style CHIP_R600 condition, fix typo in R600 blue crossbar

Signed-off-by: Mauro Rossi <issor.oruam@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>