radeon: Use request_firmware()

Loosely based on a patch by
Jaswinder Singh Rajput <jaswinderlinux@gmail.com>.

KMS support by Dave Airlie <airlied@redhat.com>.

For Radeon 100- to 500-series, firmware blobs look like:
    struct {
        __be32 datah;
        __be32 datal;
    } cp_ucode[256];

For Radeon 600-series, there are two separate firmware blobs:
    __be32 me_ucode[PM4_UCODE_SIZE * 3];
    __be32 pfp_ucode[PFP_UCODE_SIZE];

For Radeon 700-series, likewise:
    __be32 me_ucode[R700_PM4_UCODE_SIZE];
    __be32 pfp_ucode[R700_PFP_UCODE_SIZE];

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Dave Airlie <airlied@redhat.com>